view: implement separate horizontal/vertical maximize

This is a useful (if lesser-known) feature of at least a few popular X11
window managers, for example Openbox and XFWM4. Typically right-click on
the maximize button toggles horizontal maximize, while middle-click
toggles vertical maximize.

Support in labwc uses the same configuration syntax as Openbox, where the
Maximize/ToggleMaximize actions have an optional "direction" argument:
horizontal, vertical, or both (default). The default mouse bindings match
the XFWM4 defaults (not sure what Openbox has by default).

Most of the external protocols still assume "maximized" is a Boolean,
which is no longer true internally. For the sake of the outside world,
a view is only "maximized" if maximized in both directions.

Internally, I've taken the following approach:

- SSD code decorates the view as "maximized" (i.e. hiding borders) only
  if maximized in both directions.

- Layout code (interactive move/resize, tiling, etc.) generally treats
  the view as "maximized" (with the restrictions that entails) if
  maximized in either direction. For example, moving a vertically-
  maximized view first restores the natural geometry (this differs from
  Openbox, which instead allows the view to move only horizontally.)

v2: use enum view_axis for view->maximized
v3:
  - update docs
  - allow resizing if partly maximized
  - add TODOs & corrections noted by Consolatis

docs/labwc-actions.5.scd

@@ -112,11 +112,13 @@ Actions are used in menus and keyboard/mouse bindings.
 *<action name="ToggleFullscreen" />*
 	Toggle fullscreen state of focused window.
 
-*<action name="ToggleMaximize" />*
-	Toggle maximize state of focused window.
+*<action name="ToggleMaximize" direction="value" />*
+	Toggle maximize state of focused window. Supported directions are
+	"both" (default), "horizontal", and "vertical".
 
-*<action name="Maximize" />*
-	Maximize focused window.
+*<action name="Maximize" direction="value" />*
+	Maximize focused window. Supported directions are "both" (default),
+	"horizontal", and "vertical".
 
 *<action name="ToggleAlwaysOnTop" />*
 	Toggle always-on-top of focused window.

docs/rc.xml.all

@@ -298,7 +298,6 @@
       <mousebind button="Right" action="Click">
         <action name="Focus" />
         <action name="Raise" />
-        <action name="ShowMenu" menu="client-menu" />
       </mousebind>
     </context>
 
@@ -309,20 +308,30 @@
       <mousebind button="Left" action="DoubleClick">
         <action name="ToggleMaximize" />
       </mousebind>
+      <mousebind button="Right" action="Click">
+        <action name="ShowMenu" menu="client-menu" />
+      </mousebind>
     </context>
 
     <context name="Maximize">
       <mousebind button="Left" action="Click">
-        <action name="Focus" />
-        <action name="Raise" />
         <action name="ToggleMaximize" />
       </mousebind>
+      <mousebind button="Right" action="Click">
+        <action name="ToggleMaximize" direction="horizontal" />
+      </mousebind>
+      <mousebind button="Middle" action="Click">
+        <action name="ToggleMaximize" direction="vertical" />
+      </mousebind>
     </context>
 
     <context name="WindowMenu">
       <mousebind button="Left" action="Click">
         <action name="ShowMenu" menu="client-menu" />
       </mousebind>
+      <mousebind button="Right" action="Click">
+        <action name="ShowMenu" menu="client-menu" />
+      </mousebind>
     </context>
 
     <context name="Iconify">