This diff moves the logic for healing from `UnitAI` to `Heal`. It would make it easier to allow modders to let e.g. structures heal not using auras.
Downsides:
- There are two timers per entity now- We set the rotation in cmpHeal, where it doesn't belong, perhaps the one in `UnitAI`that can be removed by checkallowing the orientation in `Heal`?entities to keep facing towards their target.