Changed the type of Map to ConcurrentHashMap

Fixes the ConcurrentModificationException when using turrets shooting
multiple projectiles at once.
* ConcurrentHashMap
Changed behavour of Projectiles collisions

Ignores collisions with other projectiles and the tank itself (owner).
This also fixes the bug with projectiles not reacting to collision with
MapBounds.
Merged Turret-Projectile > imghandler