Some fixes on the release tarball task
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
Merge branch 'release-2.0.0' into devel

Conflicts:
	smc/src/enemies/furball.cpp
	smc/src/enemies/furball.hpp
	smc/src/enemies/krush.cpp
	smc/src/enemies/krush.hpp
	smc/src/enemies/turtle.cpp
	smc/src/enemies/turtle.hpp
	smc/src/level/level_player.cpp
	smc/src/objects/box.cpp
	smc/src/objects/sprite.hpp