Refactor: Improved Games and Game interfaces

- Renamed GameCollection as Games and various members for readability
- Games::currentGame() now returns a reference to the current game
- Dumped the now redundant C wrapper API

Note: Each Games collection always has a valid current Game instance
(even in ringzero mode) as the null-object pattern is employed. The
utility function de::isNullGame() should be used to determine if the
the current Game is an instance of NullGame (thus whether ringzero
mode is currently active).
Merge branch 'master' of github.com:skyjake/Doomsday-Engine