testnotebookdnd: Defer deletion to an idle

testnotebookdnd: Defer deletion to an idle

The code here was always a bit buggy: We removed the tab from
the notebook in a ::drag-data-received handler. But with
GTK_DEST_DEFAULT_DROP, that signal is emitted before we inform
the source side that the drag is finished. With its improved drag
handling, GtkNotebook now interprets this as a 'spontaneous'
removal of the tab being dragged, and cancels the drag, leading
to an unwanted cancel animation.

The easiest fix is to just defer the tab removal to an idle.