Healing Modifiers

[EliphasNUIT]

Add healing modifiers tracking like for damage modifiers.

Potential problems:

• Are all healing skills impacted by incoming/outgoing modifiers or are there flat healing skills or special kind of heals (Blood Reckoning? Lithany of Wrath?) not impacted by them.
• How to efficiently handle healings credited to the current player but that scale based on the outgoing modifiers of someone else (Soulcleave Summit). That would require every OutgoingHealingModifiers to discard such skills or/and somehow redirect it to its proper source.
• Signet of Vampirism...