Upgrade Live update operator

Created by: Durman

Problem statement

https://github.com/JacquesLucke/animation_nodes/pull/1769#issuecomment-890803459

I used the scene update handler, but due to unknown reasons this handler was always executed. We only got a "real" scene changed handler reeelatively recently, using it sounds like a good idea.

Probably instead of timer the operator could use the scene changed handler. It would help to avoid useless updates (when nothing was changed). At least it's worth to experiment.