window: Only grab/ungrab buttons/keys on X11 windows
compositor: Don't select for weird events on the COW/stage

Talking it over with Owen, we weren't sure why this was here.

At one point, we were creating a foreign stage window, so potentially
Clutter didn't select for its own events, but now we're using a standard
stage window, so this seems weird.

Why we did it on the COW, nobody knows. Maybe copy/paste bugginess?