keybindings: Fix <Super> key crash

If we exit early as not handled, then the normal process_event
handler will fire, and trigger the overlay-key binding. As that's
a special binding that doesn't have a handler, trying to trigger
that handler will crash mutter.

Instead of returning early, just check for xdisplay every time
we try to drive the X grab state machine. We really need a better
solution for this on the Wayland side.
keybindings: Don't mark a replayed keypress event as handled

This is needed so it goes to Wayland clients.