This commit centralizes event handling to where damage is actually applied
to the entity to avoid bugs that have resulted from nodamageticks,
modifications to damage after the event has been called, and similar
mishaps. This also implements new API for getting and setting of
modifications made to the damage amount actually applied to the entity.
This is done by storing the change in the damage amount as each modifier
is applied by vanilla code.
The method that actually damages the armor worn by an entity has been
relocated beneath the event called as to not apply durability loss when
the event has been cancelled.
Some types of damage are handled specially so do not end up returning an
event in handleEntityDamageEvent. To ensure we don't have problems with
these we need to check for them and simply ignore them, as they've been
handled already.
Calling this event allows plugins to react to the situation by simply
handling a normal damage event, possibly using existing code to
handle other entity damage.
Pulled from PR #1279
