Rework learning mechanics, following issue #613

Implements items 1 through 5, from issue #613.
Add feedback about focus gain to the morale screen
Learning from skill books reduces mental focus equilibrium point
Bugfix and minor cleanup to player::calc_focus_equilibrium
Minor fixes to learning's rng() calls.
No need to check for XP in the pool anymore.
Styling fix.
max_ex < min_ex produces strange results in rng()
SkillLevel::train and _exercise have increased granularity.
Fixed a bug that prevented books from giving experience at 0 skill.
Replace references to "XP" with "Focus"
Remove references to skill comprehension
Update the help text about XP and learning
Merge skill gain/morale refactor.