feat: implement Openbox-style bottom window handle and grips

Add full handle/grip assembly to the bottom of SSD window frames,
following the Openbox themerc specification for geometry and theming.
Theme parsing:
- Parse window.handle.width (handle bar height, default 6)
- Parse window.grip.width (corner grip width, default 20)
- Parse window.[active|inactive].handle.bg with Solid/Gradient support
- Parse window.[active|inactive].grip.bg (inherits from handle if unset)
- Pre-render 1px-wide fill buffers and cairo patterns for handle/grip
Scene graph (new ssd-handle.c):
- Handle assembly replaces bottom border when active, with its own
  left/right/top borders and three-segment bottom border
- Grips at left/right corners for diagonal resize (sw/se-resize)
- Center handle for vertical resize (s-resize)
- Vertical separator lines between grips and handle using border color
- Per Openbox spec, handle_width is content-only height with borders
  drawn around it (total assembly height = 2*border_width + handle_width)
Interactive visual states (grips only):
- Hover: 20% black overlay on grip content area
- Pressed: 40% black overlay with 1px inset shadow (dark top/left,
  light bottom/right) for a pushed-in 3D effect
- Dragging: 20% overlay with inset shadow maintained
- Global hover tracking (server.hovered_handle_ssd/element) ensures
  proper cleanup when cursor moves across views or to desktop
Decoration toggle cycle (ToggleDecorations action):
- New LAB_SSD_MODE_BORDER_HANDLE between BORDER and FULL
- keepBorder=true: full -> border+handle -> border -> none -> full
- keepBorder=false: full -> none -> full (unchanged)
Node types and input:
- New LAB_NODE_HANDLE, LAB_NODE_GRIP_LEFT, LAB_NODE_GRIP_RIGHT
- Integrated into LAB_NODE_BORDER/BORDER_BOTTOM containment so
  existing Border context mousebinds (Resize) work automatically
- Handle/grip descriptors resolved directly in get_cursor_context()
  bypassing ssd_get_resizing_type() for precise cursor shapes
Visibility rules:
- Hidden when maximized, shaded, or handle_width is 0
- Hidden in LAB_SSD_MODE_BORDER and LAB_SSD_MODE_NONE states
- Bottom border in ssd-border.c disabled when handle is active
Documentation:
- labwc-theme.5.scd: document all handle/grip theme properties
- labwc-actions.5.scd: update ToggleDecorations to 4-state cycle
- docs/themerc: add handle/grip default values

docs/labwc-actions.5.scd

@@ -217,13 +217,14 @@ Actions are used in menus and keyboard/mouse bindings.
 *<action name="ToggleDecorations" />*
 	Toggle decorations of focused window.
 
-	This is a 3-state action which can be executed multiple times:
-	- Only the titlebar will be hidden, borders and resize area are kept
+	This is a 4-state action which can be executed multiple times:
+	- Titlebar is hidden; borders and handle/grip remain visible
+	- Handle/grip is also hidden; only borders remain
 	- Remaining decorations will be disabled
 	- Decorations will be shown normally
 
-	By disabling the theme configuration 'keepBorder' the first step
-	will be removed and the action only toggles between on and off.
+	By disabling the theme configuration 'keepBorder' the cycle reduces
+	to toggling between full decorations and no decorations.
 
 *<action name="ToggleFullscreen" />*
 	Toggle fullscreen state of focused window.
@@ -526,9 +527,10 @@ Actions that execute other actions. Used in keyboard/mouse bindings.
 			Whether the client is tiled (snapped) to the indicated
 			region. The indicated region may be a glob.
 
-		*decoration* [full|border|none]
+		*decoration* [full|border-handle|border|none]
 			Whether the client has full server-side decorations,
-			borders only, or no server-side decorations.
+			borders with handle/grip, borders only, or no
+			server-side decorations.
 
 		*monitor* [current|left|right|<monitor_name>]
 			Whether the client is on a monitor relative to the to

docs/labwc-theme.5.scd

@@ -113,6 +113,33 @@ window.*.title.bg.colorTo.splitTo: #557485
 	Line width (integer) of border drawn around window frames.
 	Default is 1.
 
+*window.handle.width*
+	Height (integer) of the bottom handle bar in pixels. Despite the name,
+	this controls the vertical size of the handle (Openbox naming
+	convention). When set to 0, the handle/grip assembly is disabled and
+	the standard bottom border is used instead. Default is 6.
+
+*window.grip.width*
+	Width (integer) of each corner grip in the bottom handle bar, in
+	pixels. Default is 20.
+
+*window.active.handle.bg* / *window.inactive.handle.bg*
+	Texture for the handle background. Supports Solid, Gradient Vertical,
+	and Gradient SplitVertical. Default is *Solid* with color #a0a0a0
+	(active) / #c0c0c0 (inactive).
+
+*window.active.handle.bg.color* (and .colorTo, .color.splitTo, .colorTo.splitTo)
+	Colors for the handle background gradient stops. See the texture
+	documentation above for details.
+
+*window.active.grip.bg* / *window.inactive.grip.bg*
+	Texture for the left and right grip backgrounds. If not set, the grip
+	inherits from the handle background.
+
+*window.active.grip.bg.color* (and .colorTo, .color.splitTo, .colorTo.splitTo)
+	Colors for the grip background gradient stops. If not set, the grip
+	inherits from the handle background colors.
+
 *window.titlebar.padding.width*
 	Horizontal titlebar padding size, in pixels, between border and first
 	button on the left/right.
@@ -561,7 +588,19 @@ following icons should be added:
 
 # DEFINITIONS
 
-The handle is the window edge decoration at the bottom of the window.
+The handle is the window edge decoration at the bottom of the window. It
+consists of a center handle bar (for vertical resizing) and two grips at the
+left and right corners (for diagonal resizing). The handle/grip assembly
+replaces the standard bottom border when enabled (window.handle.width > 0)
+and the current decoration state includes the handle. Vertical separator
+lines of border.width thickness are drawn between the grips and the center
+handle using the window border color.
+
+When hovering over a grip element, it dims by 20%. When clicked (pressed
+but not yet moved), it dims by 40% with an inset shadow effect giving a
+"pushed in" appearance. During drag-resize, the dimming reduces to 20%
+while maintaining the inset shadow. The center handle does not receive
+these visual effects.
 
 # SEE ALSO
 

docs/themerc

@@ -58,6 +58,19 @@ window.inactive.shadow.size: 40
 window.active.shadow.color: #00000060
 window.inactive.shadow.color: #00000040
 
+# window bottom handle and grips
+window.handle.width: 6
+window.grip.width: 20
+window.active.handle.bg: Solid
+window.active.handle.bg.color: #a0a0a0
+window.inactive.handle.bg: Solid
+window.inactive.handle.bg.color: #c0c0c0
+# Grips inherit from handle if not set explicitly
+# window.active.grip.bg: Solid
+# window.active.grip.bg.color: #a0a0a0
+# window.inactive.grip.bg: Solid
+# window.inactive.grip.bg.color: #c0c0c0
+
 # Note that "menu", "iconify", "max", "close" buttons colors can be defined
 # individually by inserting the type after the button node, for example:
 #