WindowServer+LibGUI: Allow switching windows in/out of fullscreen mode

You can now call GWindow::set_fullscreen(bool) and it will go in or out
of fullscreen mode.

WindowServer will also remember the previous window rect when switching
to fullscreen, and restore it when switching back. :^)

Servers/WindowServer/WSClientConnection.h

@@ -84,6 +84,7 @@ private:
     void handle_request(const WSAPIDismissMenuRequest&);
     void handle_request(const WSAPISetWindowHasAlphaChannelRequest&);
     void handle_request(const WSAPIMoveWindowToFrontRequest&);
+    void handle_request(const WSAPISetFullscreenRequest&);
 
     void post_error(const String&);