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

src/ssd/ssd.c

@@ -35,7 +35,7 @@ ssd_thickness(struct view *view)
 
 	struct theme *theme = view->server->theme;
 
-	if (view->maximized) {
+	if (view->maximized == VIEW_AXIS_BOTH) {
 		struct border thickness = { 0 };
 		if (!view->ssd_titlebar_hidden) {
 			thickness.top += theme->title_height;
@@ -226,7 +226,8 @@ ssd_update_geometry(struct ssd *ssd)
 			ssd_extents_update(ssd);
 			ssd->state.geometry = current;
 		}
-		if (ssd->state.squared_corners != ssd->view->maximized) {
+		bool maximized = (ssd->view->maximized == VIEW_AXIS_BOTH);
+		if (ssd->state.squared_corners != maximized) {
 			ssd_border_update(ssd);
 			ssd_titlebar_update(ssd);
 		}