Enable submeshes to be separate GameObject's again

[blgrossMS]

Our application would like the ability to toggle submesh visibility similar to Babylon.js. It would be good if there was an import option added to be able to choose whether submeshes are combined into one mesh, or treated as separate meshes/GameObjects. Unity also has the ability to choose this in asset import settings.

[blgrossMS]

To be clear, this is a regression from previous functionality and does not allow us to expose the ability to turn on and off parts of the mesh in our viewer. In order for our customers to have this functionality, they will have to reauthor their assets, or author them differently.