Open 200: Ability to import and overlay multiple models in a project

[OlegMoshkovich]

As a user, I would like to have the ability to import multiple IFC models into a project, and overlay them onto one another to achieve a holistic view of the project. Due to the data-rich nature of IFC models, not all models need to be actived with all elements accessible at all times. Instead, it is sufficient to have a ghosted view of each model that can be fully loaded into the session when required. All models must be accurately positioned in the correct location, and there should be a top-level view in the project tree that contains all of the model references.

Figma screens

[OlegMoshkovich]

Previous discussion is captured in issue #248