GtkWidget: Improve drag-leave and drag-data-received docs.

GtkWidget: Improve drag-leave and drag-data-received docs.

    * gtk/gtkwidget.c: drag-leave signal: Document that it is called before
      drag-drop.
      drag-data-received signal: Document that it is up to the application
      to know why the data was requested (e.g. drag motion or drop).
    * demos/gtk-demo/toolpalette.c: interactive_canvas_drag_drop():
      Do not transform the drop_item created in the drag-motion handler.
      Instead caused drag-data-received to be called, remembering why,
      and create a new item there.
      interactive_canvas_drag_leave(): Remove the idle-handler hack,
      now that we do not need to keep the drag-motion drop_item alive until
      the drop.

I noticed that this patch was sitting in bug #605611 from 2009
though it had been approved. I do not remember much about why I
created it.