On wayland the DnD surface must be created early when starting the drag
operation, so offer API for GTK+ to get the GdkWindow used as a DnD
surface on the drag operation.
https://bugzilla.gnome.org/show_bug.cgi?id=697855
The wl_data_source is retrieved from the selection object for the DnD
selection, and used to initiate a drag. When the drag is finished, a
button release or touch end event is synthesized to finish the DnD
operation after the compositor grab is gone.
https://bugzilla.gnome.org/show_bug.cgi?id=697855