Ported monster tank madness to lua

~~It would be nice, if someone could already have a look at this (style nits?),
although this is not completely finished yet (see todo).~~ See below.

[This](https://github.com/abcdefg30/OpenRA/commit/f5fe14701284c88ecdf66b5f8fb3f5db24a7296e#diff-da0b70bbc22b6b4af2fbe96dd5c5a8b7R19) set to true can help testing.
pchote pointed out a problem with the triggers where the trigger function would get called multiple times if several actors entered or left it during the same tick. So you need a guard variable to circumvent this (although in many of these places it probably doesn't matter, eg. the beach trigger). It does when you're spawning actors, though. See gdi04a/b for examples.