Powerup: Iceflower improvements

Generalized code for changing frozen badguy sprite to iced, and added a default behavior for freezable badguys without iced graphics.  Frozen badguys no longer unfreeze on touch.  Added ice vulnerability to forest badguys.
Note: there are some mild bugs as a result of this change, which should be fixed in subsequent improvements to make Iceflower a more viable powerup.