Beta Branch: Setting agents loose

Probably the last feature which will go into V18, after this it should just be bugfixing for these new features and a few minor tweaks

-Agents can now be "set loose", causing them to become AI-controlled. They'll wander around and try to infiltrate and spread shadow (regardless of their class). They're weaker and worse than they would be under your control, but it's a better use of them than simply killing them off

-Talking of, agents now have an ability called `apoptosis' which kills them off

Both these free up an enthrallment slot, the first costs you an enthrallment use (to avoid it being OP, it's not meant to be a full strategy, just a cool thing to do with a unit which has served its purpose).

Quite easy to do, actually, because this AI functionality was already implemented, for testing purposes (the automatic tester down the bottom left of the main menu), so when EuchreJack suggested it I figured I could spare a few hours to copy-and-past some code.