Right now the Synchronizer for selctors seems to work for pointer events.

• make it work for arrow key events too, so that we can sync selectors between different renderers (and therefore different plot areas)
• have two modes
  • one where it just sychronizes the deltas (this is how it is currently implemented) but Selectors can all be in different positions.
  • another mode where it sychronizes the absolute position of the selectors, this is useful for moving things in sync across different renders (i.e. different plot areas)