wayland: Implement drag/source side of selections

wayland: Implement drag/source side of selections

This has been made to work similarly to X11, requests for the data device
contents are notified through GDK_SELECTION_REQUEST events, the data stored
in the GDK_SELECTION property as a reaction to that event is then stored
into the wayland selection implementation, and written to the fd when
requested/available.

https://bugzilla.gnome.org/show_bug.cgi?id=697855