Remove active/inactive flag from game views.
Remove extra new line.
Add accessor for views in GameViewManager.
Clean up transition logic in AbstractGameView.

GameView transitions should get highest priority when they are exiting,
not when they are covered. Additionally, if a view is covered it should
begin to transition off (and back on when uncovered).

The return value for updateTransition has also been flipped, which seems
to make more sense.
Update documentation for AbstractGameView#exit.
Document GameViewManager#getViews.
Add exit methods to game, view, and systems.
Remove abstract keywords from interface.
Remove active/inactive flag from game systems.
Update documentation for GameViewManager#update.
Remove unused private field.
Clear collections after exiting.
Remove ability to change system process order.

Systems must now declare their (immutable) process order at the time of
registration with a manager. This seems to cover most cases.
Add tests for exit method.