Freeze break request: fixing copy-paste under Wayland

[Matthias Clasen <matthias clasen gmail com>]

I've been running a Wayland session for the last few days. One of the
rougher points of this experience that drove me  back to X was that
copy-paste didn't work between applications.

The following three commits by Carlos fix this:

https://git.gnome.org/browse/gtk+/commit/?id=a59fd4a9ad3f7c60ada2acd6809ad2173b5107fd
https://git.gnome.org/browse/gtk+/commit/?id=d5160bb950a736d54cedcd2292af1311e5112c04
https://git.gnome.org/browse/gtk+/commit/?id=6c8c37abbb641f3959ab45a5de4adb2aeb36f653

I'd like to cherry-pick these onto the gtk-3-16 branch before .0.
Given that they only touch the Wayland backend, the risk is fairly low.