Introducing model descriptions and instances

[xiyuoh]

This is a continuation of Reuben's PR #230, opening a new one to accommodate push restrictions.

This PR:

• Introduces the concept of model descriptions and instances, where a model description provides the information about a model (asset source, scale, etc.), and a model being spawned on site is an instance of a particular model description. This allows users to make changes to a group of model instances with the same model description at one go. This is helpful for sites with multiple identical models.
• Allows users to create different scenarios with a scenario tree, where they can edit and switch between variants of the same environment with the ability to undo or redo changes without affecting the root scenario.
• Provides a configurable property plugin that enables users to add new properties and configuration widgets, such as mobile robot tasks for instances and differential drive properties for descriptions.

[xiyuoh]

Thank you both @luca-della-vedova and @mxgrey for the extremely detailed and insightful review. I've addressed the comments and am ready for another round of lookthrough.

[mxgrey]

After spending all this time reviewing the quality of the implementation in terms of its code, I finally spent some time playing around with the actual capabilities that are introduced.

It's honestly very cool. Seeing changes in parent scenarios affect all child scenarios simultaneously, and being able to make changes within child scenarios and revert them... this is a very powerful feature.

Thanks @reuben-thomas and @xiyuoh for all the tough work figuring out how to wrangle the entities and components to make this possible. And thanks @luca-della-vedova for kicking off the review with so much valuable feedback.

There's no shortage of things to follow up on from here, but this is an amazing start.