The SpeedPowerUp now adjusts minSpeed aswell
Added a new image for the tank and turret

It's currently not colored at all. I don't know if we intend to have 4
(or more) different colors for the tanks.
The spawn shield should now be removed properly
Removed unused imports
Reworked the shockwaveprojectile (sort of)

The initial projectile now detonates on walls and other tank instead of
just be destroyed. This should make the weapon a little bit more useful
(and fun). Also, the initial projectile no longer deals any damage. The
damage of the shockwave itself might have to be tweaked since it's a
little overpowered at the moment. A solution might have to be
implemented where there's a maximum damage that the shockwave can do to
a single tank.