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/WSCompositor.h

@@ -61,6 +61,7 @@ private:
     DisjointRectSet m_dirty_rects;
 
     Rect m_last_cursor_rect;
+    Rect m_last_dnd_rect;
     Rect m_last_geometry_label_rect;
 
     String m_wallpaper_path;