Merge Threads into GameThread, Introduce interfaces for Entities.

Background now inherits Entity, too.
Fixed imports all over the place.
Fix sleeping, now everything runs smooooooooothly.
Merge mit erster Fensterkollision und Enemy-Implementation.

Conflicts:
	src/de/teamteamteam/spacescooter/Main.java
	src/de/teamteamteam/spacescooter/background/StarBackground.java
	src/de/teamteamteam/spacescooter/entities/Player.java