wayland: Store requested target on GdkWaylandSelection
wayland: Store requested target on GdkWaylandSelection

It doesn't make a lot of sense to have this stored as data offer data,
rather together with the source_targets array, which is what we're
poking here in the end.

https://bugzilla.gnome.org/show_bug.cgi?id=758713

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita