Master issue for global event handling suggestions and improvements

An assortment of suggestions to event handling. Codependent on #321 and #325.

- [ ] Instead of having a separate function for every event, have a single function and the event passed in as a constant (**maybe**)
- [ ] The ability to programmatically call an event handler for both events and menu actions; or alternatively, a way to specify that they should be called when programmatically changing something (**maybe**)
- [ ] Likewise, but for uiQuit() (**maybe**)
- [ ] Drop uiAreaHandler and switch to event callbacks for it, passing the data argument normally instead of extending the structure; or alternatively, having the data argument be part of uiAreaHandler itself (**maybe** for the latter)

This topic will be unlocked when I merge existing issues in.