Fixed Collisions, Implemented Immovable Crate

Collisions, even those created by rotation, is now managed. objects
slowly get moved apart when they overlap, if they have some velocity
their movement to get where they are is recursively undone.

Immovable Crates now don't move ever.

Tail-whip bug
Some problem with react(...) of tanks causing them to behave weirdly
when one tank is being propelled and turns into another tanks which
isn't being propelled. Short term solution: reduced the effect of
react(...) for tanks, it is still in place though, allowing tanks to be
pushed around by other tanks.