menu: support titles

...defined by `<separator label="">`.

Also add the theme option `menu.title.bg.color: #589bda`

The following will be added in separate commits
- menu.title.bg.border.color: #7cb6ec
- menu.title.text.color: #ffffff
- menu.title.text.justify: center

docs/labwc-menu.5.scd

@@ -25,7 +25,7 @@ The menu file must be entirely enclosed within <openbox_menu> and
   <!-- A submenu defined elsewhere -->
   <menu id="" />
 
-  <!-- Horizontal line >
+  <!-- Horizontal line -->
   <separator />
 
   <!-- An inline submenu -->
@@ -33,6 +33,9 @@ The menu file must be entirely enclosed within <openbox_menu> and
     ...some content...
   </menu>
 
+  <!-- Title -->
+  <separator label=""/>
+
   <!-- Pipemenu -->
   <menu id="" label="" execute="COMMAND"/>
 
@@ -63,6 +66,11 @@ The menu file must be entirely enclosed within <openbox_menu> and
 *menu.separator*
 	Horizontal line.
 
+*menu.separator.label*
+	In a "separator" element, the label attribute transforms the separator
+	from a horizontal line to a menu title (heading) with the text specified
+	by label in it.
+
 *menu.execute*
 	Command to execute for pipe menu. See details below.
 

docs/labwc-theme.5.scd

@@ -174,6 +174,10 @@ elements are not listed here, but are supported.
 *menu.separator.color*
 	Menu separator color. Default #888888.
 
+*menu.title.bg.color*
+	Menu title color. Default #589bda.
+	Note: A menu title is a separator with a label.
+
 *osd.bg.color*
 	Background color of on-screen-display. Inherits
 	*window.active.title.bg.color* if not set.

docs/themerc

@@ -62,6 +62,7 @@ menu.separator.width: 1
 menu.separator.padding.width: 6
 menu.separator.padding.height: 3
 menu.separator.color: #888888
+menu.title.bg.color: #589bda
 
 # on screen display (window-cycle dialog)
 osd.bg.color: #e1dedb