Support simultaneous viewing of multiple models

Support for simultaneous viewing of multiple models in the viewer at once is required for augmented reality experiences. Consider the example of a "try-and-buy" experience where the user would like to see how multiple pieces of furniture look in their living room. Modular sofas are an example whereby different components can be combined in myriad configurations based on the dimensions of the space and the tastes of the customer. The creator of the experience should be able to add all of the sofa module models to the package. The implication is that once added to the viewer, the user would need to be able to select the bounding box for each model individually in order to manipulate it.

This is related to issue #481 in that there need to be multiple models packaged with the experience and available for selection, but this differs in that this use case requires multiple models to be simultaneously present in the viewer then selectively manipulated..