Rework learning mechanics, as per issue #613

First pass at reworking the XP/learning mechanics. Implements items 1 through 5, from issue #613.

Still needs a bit more work before a proper submission (such as UI updates), but I think that testing/balance feedback at this phase would be useful.

Not template-compatible, and only partially save-compatible, but the next couple batches of changes should be fully compatible.

EDIT: Also added in items 6 and 8 from FunnyMan's proposal: the bit about books reducing mental focus, and showing focus gain in the morale screen. I ended up going from 100 focus to 22 by training tailoring from 0 to 1 via book, which is pretty impressive. It'll be interesting to see if reading's been nerfed too far or not.

EDIT2: This actually works pretty well - I'm surprised, I thought we'd need to tweak stuff a bit more. Props to FunnyMan for coming up with good numbers the first time around.