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. :^)
