Fix game over screen.
Enemies can damage the player character.
Enhance graphics with the ability to draw preformed rectangles.
Game Objects temporarily become passive after damaging another object.
Better threading performance and Game Objects return to passive states according to their types.
Refactor for clearer attacking methods
Game objects only damage other game objects of the right type (class).
Each game object delivers its own specific damage amount when attacking.
Healing not used so removed.
Game objects only attack once regardless of how many overlapping attack and damage zones.
Damage to a game object is highlighted by a red circle appearing on screen at the point of impact.
Merge branch 'combat'