Include/Hide model instances in scenarios

[xiyuoh]

This PR addresses one of the issues in #254 to include/hide model instances from scenarios rather than adding/removing them entirely.

Major changes:

• No more added_instances, moved_instances and removed_instances; each Scenario stores instances with data about whether their poses have been moved from any parent scenario, and whether they are hidden in the current scenario.
• A collapsible widget listing all model instances (categorized by the affiliated model description), through which users can interact with each model instance:
  • Include or hide the model instance in the current scenario
  • Reset the model instance pose if it has been moved from any parent scenario
  • Remove the model instance from the site entirely (alternatively the Del key works on selected model instances too)
  • Displays the model instance's pose in the current scenario
• Removed deletion filter for model instances, now that we've moved to a include/hide approach rather than add/remove, so there's no longer a need to filter out deletion events based on whether the current scenario is a root or child.