(Issue #578) Bonus HP & MP Effects From Items to affect actual hp/mp

See issue: https://github.com/clintbellanger/flare-engine/issues/578 

The changes should allow for HP/MP bonus items to preserve hp/mp ratios when added or removed (as per Stefan's solution in the above issue discussion)

Submitted on behalf of lp150189 

EDIT:

Working on understanding and implementing buffs granted from Powers to operate similarly (preserve ratios).