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The wayland specific clipboard functions have been replaced by something
more similar to the hooking the win32 backend does, which allows for just
using the default GtkClipboard code in GTK+. As a consequence, the
wayland-specific GtkClipboard implementation is now gone.
https://bugzilla.gnome.org/show_bug.cgi?id=697855
