DeltaTime & collision smoothing. Small naming tweaks.

* Added DeltaTime calculation, which smooths things out substantially.
* Renamed player/waiting_player to player_one and player_two.
* First draft of 'player quit' cleanup - not yet working.
* Make canvas square to ease reasoning about hit detection/canvas drawing
misalignment.
* Add a collision cooloff to the Ball so that it doesn't trigger many
sequential hits on a single paddle collision.