Script climbing plant in lvl_3
Support random level entries.

If a level exit or return exit uses a level entry, and that entry
exists multiple times, the entry used will be randomly selected.
Remove check on Is_Basic_Sprite() for massivity toggling. Fixes #63.

Objects that don’t want to be toggled have to overwrite Set_Massive_Type()
now accordingly.
Merge branch 'feature-randomentry' of git://github.com/brianvanderburg2/SMC into devel