Behavior tree cleanup

[ghost]

The Behavior Tree files have received lot of changes since 0.52, and even if the default.bt was split in 2, things are slowly but steadily becoming annoying big, and I think the BT can be cleaned much (as in, removal of syntactic sugar, splitting files more maybe, refactoring conditions, etc).

[ghost]

I think this would be an important task, but at this point it would likely be a rewrite of the .BT files.
While I think this is desirable, it could make sense to 1st describe generally how bots should behave in plain english (that is, write specs), and to have describe that way more than one behaviors, since in the past year, only default behavior received improvements.

When this would be done, it would enable writing the necessary C++ code to support the goal, instead of random patching. See Unvanquished/Unvanquished#1727