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. :^)

Libraries/LibGUI/GWidget.h

@@ -254,6 +254,7 @@ protected:
     virtual void leave_event(CEvent&);
     virtual void child_event(CChildEvent&) override;
     virtual void change_event(GEvent&);
+    virtual void drop_event(GDropEvent&);
 
 private:
     void handle_paint_event(GPaintEvent&);