LibGUI+WindowServer: Start fleshing out drag&drop functionality

This patch enables basic drag&drop between applications.
You initiate a drag by creating a GDragOperation object and calling
exec() on it. This creates a nested event loop in the calling program
that only returns once the drag operation has ended.

On the receiving side, you get a call to GWidget::drop_event() with
a GDropEvent containing information about the dropped data.

The only data passed right now is a piece of text that's also used
to visually indicate that a drag is happening (by showing the text in
a little box that follows the mouse cursor around.)

There are things to fix here, but we're off to a nice start. :^)

Servers/WindowServer/WindowClient.ipc

@@ -27,4 +27,9 @@ endpoint WindowClient = 4
     WM_WindowRectChanged(i32 client_id, i32 window_id, Rect rect) =|
 
     AsyncSetWallpaperFinished(bool success) =|
+
+    DragAccepted() =|
+    DragCancelled() =|
+
+    DragDropped(i32 window_id, Point mouse_position, String text) =|
 }