Resolve Handle_Collision_Box todo.

Instead of centrally handling how enemies react to a hit from
a box below it inside cBox, we now delegate that to a call to
Handle_Collision_Box() so each enemy can decide on its own
how to act.
Treat cShell like cTurtle for death check. Fixes #78.
Merge branch 'fix-loose-shell-box-behaviour' into release-2.0.0