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/xdg.c

@@ -226,7 +226,8 @@ handle_request_maximize(struct wl_listener *listener, void *data)
 	if (!view->mapped && !view->output) {
 		view_set_output(view, output_nearest_to_cursor(view->server));
 	}
-	view_maximize(view, xdg_toplevel_from_view(view)->requested.maximized,
+	bool maximized = xdg_toplevel_from_view(view)->requested.maximized;
+	view_maximize(view, maximized ? VIEW_AXIS_BOTH : VIEW_AXIS_NONE,
 		/*store_natural_geometry*/ true);
 }
 
@@ -503,12 +504,10 @@ xdg_toplevel_view_map(struct view *view)
 			position_xdg_toplevel_view(view);
 		}
 
-		if (!view->fullscreen && requested->fullscreen) {
-			set_fullscreen_from_request(view, requested);
-		} else if (!view->maximized && requested->maximized) {
-			view_maximize(view, true,
-				/*store_natural_geometry*/ true);
-		}
+		set_fullscreen_from_request(view, requested);
+		view_maximize(view, requested->maximized ?
+			VIEW_AXIS_BOTH : VIEW_AXIS_NONE,
+			/*store_natural_geometry*/ true);
 
 		/*
 		 * Set initial "current" position directly before