xwm: support maximizing xwayland windows

This patch adds the maximize button to the window frame for the windows
which set the MWM_DECOR_MAXIMIZE hint, and it wires it with the shell
via a new method in weston_shell_interface.
Additionally, it also listens for the wm hints coming from the client,
but it doesn't support maximizing a window only vertically or horizontally.
The window will be maximized only when both directions are maximized.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

src/compositor.h

@@ -121,6 +121,7 @@ struct weston_shell_interface {
 	void (*set_window_geometry)(struct shell_surface *shsurf,
 				    int32_t x, int32_t y,
 				    int32_t width, int32_t height);
+	void (*set_maximized)(struct shell_surface *shsurf);
 };
 
 struct weston_animation {