More work on collisions - working version
The following collisions now work:
- Enemy bullets hitting the player with proper damage calculation taking into account enemy's attack, bullet's attack, and player's defense.
- The player running into an enemy (or vice versa).
 
The following still need work:
- Player bullets hitting the enemy.
 
Other changes:
- Organization of SpaceInvaders.java: loadScripts(), continueStepping()
- New static EntityGroup methods: checkCollision() and calculateDamage()
- Max HP for player works now, in preparation for saving.
- Dialog boxes commented out.
- Freeze section commented out.
- Minor changes to upgrades package so the project compiles.