interactivity, continued #100

followup from #39

ideas:

• bidirectional as a kwarg, it's quite cluttering when you want to do contours -> temporal stack, contours -> temporal-single, rcm-contours -> temporals etc. and the other way too. It would be valid only for links that have a 1-1 mapping, like line collection colors etc.
  • done in More interaction #106
• add all the pygfx events #110
• Think about whether euclidean mapping between graphics should be a default feature, this will probably be quite common. Think of a way to implement this nicely. On the API side perhaps a kwarg of indices_mapper = "euclidean".

from #39:

Allow multi selection too such as:

• ctrl + click contours, heatmap rows, etc.
• draw a shape with freehand or a rectangle to select multiple datapoints
• allow changing features of datapoints that are not selected, the inverse of the set of selected datapoints
• For most graphics we can probably define new events such as "hover". For graphics such as line we can allow this to be triggered either by hovering on the actual line, or by an area enclosed by the line.

@clewis7 thoughts?