Idea: Replace linked variables with selected object variable

[amacneil]

Our 3d panel linked variables feature is rather complex. There is a lot of UI for it in the 3D panel, and a similar feature does not yet exist for the image panel.

I believe we could replace linked variables by simply exposing an automatic selected object variable.

• In Node Playground, we could pass this specifically into scripts.
• In Plot etc, we might need to allow nested fields of the selected variable somehow, e.g. /tracked_objects.tracks[$selected.track_id].velocity