Merge Character Join, Leave, Change Expression into a single "Character" event

### Describe the project you are working on

Game with Ace Attorney-type of animated characters with a lot of movement

### Describe the feature / enhancement and how it helps to overcome the problem or limitation

Currently, Join/Leave/Change Expression events are not handled the best, because out of all of them only the Join event has the incredibly useful preview option.

Instead, all 3 events could be merged into 1 "Character" event, and the "Leave" behavior could be replicated when you set the expression to "None".

This also would allow easier implementation of transition animations, and for "Leave" transitions it could simply play the transition animation in reverse.