Combat: Refactor CombatActions to support multiple concurrent actions

[robzienert]

In the current combat system, only one ability can be active at a time because the CombatAction component is bound to its source entity.

This should be refactored so that the ActionProcessingSystem adds the CombatAction component to the ability entity instead, and attach the source to the CombatAction, as opposed to the CombatAction having a reference to the ability.