Added ability to remove items when hit

No scoring or time limit yet, but items that are hit are now removed.
Added setMaxSpeed() to set ShaftAction's maximum speed.
Decrease speed of shaft when hitting "BOMB"

Shaft's speed now reverts to the base speed when hitting ItemType.BOMBs.
This is like the original game if I remember correctly. Achieved by
storing the speed set via setSpeed() and setting the speed to the stored
base speed whenever the shaft hit a BOMB. Added setSpeedIncrease as well
so I can replace that ugly 1.25f in ShaftAction.