Closes #3157

## Changes:

- The action callback of Trigger now returns the entity that triggered it.
- General cleanup of Trigger class
  - `target` is now 100% separate from `filter`, because that's what the functionality described in the documentation implied (to me).
  - Converted all the different "default value fallbacks" to a single setup.
  - Trigger used `Entity` and `Actor` interchangeably. Rewrote it to be 100% Entity (Although Entity doesn't have any collision mechanics build in, so an argument could be made to go for 100% Actor)