Add support for custom "preview scene" selection when previewing an asset in the asset view

[adriengivry]

Problem this feature should fix
When previewing certain assets (i.e. PBR materials), the preview might not reflect real life scenarios, as PBR requires auto-exposure, tonemapping, and a specific light setup to work well.

Expected solution
Overload should ship with a default "preview scene" that would be used in the asset view. The asset view could have a drop down, or a "preview scene" option, so that the user can select which context to put their asset in when previewing.

This work could also allow our PBR material to become the new standard.