LibGUI+WindowServer: Mark minimized window backing stores as volatile

WindowServer will now send out a WindowStateChanged message to clients
when one of their windows is minimized.

This is then forwarded to the GWindow, which will try to mark its
underlying window backing store as volatile.

This allows the kernel to steal the memory used by minimized windows
in case it starts running low. Very cool! :^)

Servers/WindowServer/WindowClient.ipc

@@ -12,6 +12,7 @@ endpoint WindowClient = 4
     KeyUp(i32 window_id, u8 character, u32 key, u32 modifiers) =|
     WindowActivated(i32 window_id) =|
     WindowDeactivated(i32 window_id) =|
+    WindowStateChanged(i32 window_id, bool minimized) =|
     WindowCloseRequest(i32 window_id) =|
     WindowResized(i32 window_id, Rect old_rect, Rect new_rect) =|