[#580] Add prepareAbilities actor hook

[roth-michael]

Closes #580
Nothing changes for Adversary actors, since we never consider any bonus for abilities to them. Can change that, if desired. For Hero actors:
prepareBaseData now calls _prepareBaseAbilities, which is the same as it was before but doesn't clamp ability.value and doesn't include ability.bonus
prepareDerivedData now calls the prepareAbilities actor hook, then _prepareAbilities, which adds ability.bonus to each ability.value and clamps it between 0 and 12.
This means one can now provide bonuses to ability scores for Hero actors by either:

• Targeting system.abilities.[ability].bonus ( or .value, as before) via AE
• Modifying abilities.[ability].bonus or .value in the prepareAbilities hook.

[aaclayton]

This looks good, but I'm worried about order of operations and whether we can wait to compute total ability scores until prepareDerivedData. I suspect that there are some things that happen during prepareEmbeddedDocuments that currently reference and depend on the values of ability scores.

I have not looked closely, just a hunch. Do you foresee any issues with computing a final ability score after embedded items are prepared?

[roth-michael]

Ah, yeah, you're right. Specifically preparing defenses (physical and save) and preparing skills. Not sure what the best way to work around this is. Can discuss later with the other PR concerns.