Two effect-related issues

[NinthDesertDude]

Clicking cancel on the effect preview dialog does not cancel it (it's just treated the same as clicking OK right now). The old effect token needs to be tracked so it can be reverted to.

The effect preview dialog should not preview on repeated undo and redo. Perhaps it could be done with a timer. I thought about doing it on mouse move, but it's possible to undo/redo and then just click to start drawing.