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/WindowServer.ipc

@@ -71,4 +71,6 @@ endpoint WindowServer = 2
 
     GetWallpaper() => (String path)
     SetWindowOverrideCursor(i32 window_id, i32 cursor_type) => ()
+
+    StartDrag(String text, i32 bitmap_id, Size bitmap_size) => (bool started)
 }