Better handle keyboard input sent to a non-focused surface

[wmww]

There's a mismatch between Mir's keyboard input model and Wayland's. In Mir, keyboard events get sent to surfaces, and in Wayland they get sent to seats. This means that it's theoretically possible for Mir input events to go to surfaces that have not been given keyboard focus, while this is not possible in Wayland. To bridge this gap, we have logic in WlKeyboard::key that just-in-time switches focus to the surface that's receiving the events.

This logic was always broken as explained in #1625. That PR seems to improve behavior, but doesn't appear to be completely correct. We should take another look at this whole problem, and make sure everything is doing what it should.