Add badguy property to activate/deactivate badguys in groups

[mstoeckl]

Putting two jumpy badguys in the same group will ensure they are activated/deactivated at the same time; as a result they will maintain synchronization, no matter how quickly Tux approaches them.

Related level design issue: #2318 .

I'm not certain this is the best approach to solve the synchronization problem, but it is the simplest thing I could think of.

I've only edited one level ("Above the Arctic Skies") to use this feature, and can edit more if people wish. (Although where exactly to use this feature is a matter of taste.) I've only tested this with jumpy enemies and floating snowballs so far; while I don't expect other badguy types to break with this, I don't know the code well enough to be certain. Names are hard to change after the fact, so suggestions for improvements are welcome.

[mstoeckl]

1. I've rebased this to fix a merge conflict. This PR is not very important and is somewhat experimental, so I may forget about future conflicts; please reply if you ever want me to rebase it.

2. Looking at most of the existing levels, I only see a few places where load-group would be helpful, probably because existing levels have been designed to avoid sections whose difficulty depends on loading patterns. (Also, features that might need load-group (dense groups of enemies, bundles of flames, steep flying snowball step ascents, and neighboring jumpies) generally reduce the number of safe spots and force the player to keep moving, which would make levels unusually hard for world1/world2.)