Basic collision detection attempt, all logic in event definition.
Outline of humanistic algorithm for event collision detection.
Detect collision by looping using #all? Not finding fields where collisions occur yet.
Display overlapping mouse event fields in collision exception text.
Move collision logic into MouseEvent#collision, and short-circuit when propertiy set only in one event.
Remove old exception message.
Rename #collision to #collide_with
Remove collision between move text and select object.
Clean up comments and formatting.
Remove MouseEvent#==

No good way to define actual equality test. Collision is a bit different.
Edit comments.
Basic detection of mouse event collisions.