WindowServer+LibGUI: Add a way to get notified at display refresh rate

This patch adds GUI::DisplayLink, a mechanism for registering callbacks
that will fire at the display refresh rate.

Note that we don't actually know the screen refresh rate, but this is
instead completely driven by WindowServer's compositing timer. For all
current intents and purposes it does the job well enough. :^)

Servers/WindowServer/WindowClient.ipc

@@ -35,4 +35,6 @@ endpoint WindowClient = 4
     DragDropped(i32 window_id, Gfx::Point mouse_position, String text, String data_type, String data) =|
 
     UpdateSystemTheme(i32 system_theme_buffer_id) =|
+
+    DisplayLinkNotification() =|
 }