Composite pattern with strategy

- Added compositie for enemies
- Enemies can be "wave" of enemies or single enemy
- strategy added for enemy movement

@HallCat @SheepishMonster 
Don't know if there's a need for this? 
Would every enemy in the wave actually need to have their health updated?
I guess there could be a situation where they would all die or get a boost or something, but, just wondering what you're thinking. Same for SetPower?

(Or was it because EnemyInterface had them at the time?)