Move `PhocKeyboard` to `PhocInputDevice` too

This brings all of our input device related GObjects in line so we don't have N half finished things and starts to entangle PhocSeat a bit. Based in !303 (merged) and !305 (merged)

I'm marking this as draft /draft since I want to double check things myself tomorrow but this way I can grab debs to run.

EDIT: again splitted with minimal reindentation so should be o.k. to review via

git log -p --reverse master..HEAD