Commonize `XEventTransition` system code

[azriel91]

In GitLab by @azriel91 on Aug 23, 2019, 11:09

There is an emerging pattern for transition systems, of the form:

• XyzTransitionSystem is a transitions system that reads XEvents, and determines which sequence to transition to based on XTransition.
• There may be multiple XTransitions, which may be skipped if their (optional) requirement is not met.
• The requirements need to read other Component values from the entity to determine if they are met.

Table:

System	Subscribed event channel	Transition type
CharacterControlTransitionsTransitionSystem	ControlInputEvent	CharacterControlTransition
HitEffectTransitionSystem	HitEvent	HitTransition
HittingEffectTransitionSystem	HitEvent	HittingTransition
CharacterHitEffectSystem	HitEvent	CharacterHitTransition

It is likely this pattern is going to be further used, and so generic code should be written to reduce the duplicated semantics of these types and logic.