wayland: Add support for custom surfaces

Allow to set a GdkWindow to use a custom surface instead of a
wl_shell_surface. It allows to register the surface as a custom type
with some Wayland interface.

https://bugzilla.gnome.org/show_bug.cgi?id=695861

gdk/wayland/gdkwaylandwindow.h

@@ -47,6 +47,8 @@ GType                    gdk_wayland_window_get_type             (void);
 struct wl_surface       *gdk_wayland_window_get_wl_surface       (GdkWindow *window);
 struct wl_shell_surface *gdk_wayland_window_get_wl_shell_surface (GdkWindow *window);
 
+void                     gdk_wayland_window_set_use_custom_surface (GdkWindow *window);
+
 G_END_DECLS
 
 #endif /* __GDK_WAYLAND_WINDOW_H__ */