Fireballs disappear when touching platforms

Added collision detection for playerinteractor-platform collisions, and
a method collideWithStatic() which does nothing for wizards and
powerups but destroys the fireball when it touches platforms.
Increasing speed with difficulty

The speed now increases slightly over time. Exact pacing is not final,
feel free to suggest ideas for when and how speed should increase.
Speed boost powerups still boost speed for a little while, but ordinary
incrementations still happen.