Battle City Alpha 3

New version, written from completely from scratch. There are still code improvements that need to be made before continuing on.

Major differences between Alpha 2 and Alpha 3 builds:
- Game State Manager (Main menu and Game screen)
- Maximum enemy tanks on screen increased from 3 for both single and multiplayer to 4 for single and 8 for multi (may reduce to 6)
- Brick walls destructible, instant game over when castle is destroyed
- Various code improvements