Maintenance updates; new functionality

- Removed unused enums;
- Added PlayerType enum;
- Removed redundant bool StateManager.HDEnabled;
- Added List<Ship> ActiveShips and Guid PlayerID to StateManager; used
to track ships in a network game;
- Moved Ships classes to Ships\Allies
- Created Ships\Enemies folder
- Minor code cleanup in GameScreen.cs
Administrative update; missed in last commit