Blender Git Loki

Support object preview thumbnails, generated on asset creation

To generate the preview, a temporary main data-base is created (think of this
as a virtual .blend within the current .blend) with a new scene and the object
to generate the preview for. It's rendered with a camera from the front, using
the "Camera Fit Frame to Selected" logic.
This should be a simple solution requiring no further setup that should work in
most cases. If needed we can have a way to set up the preview differently.

To see it in action:
* In the Outliner context menu for an object run ID Data > Create Asset
* Save the file
* Open a different file (so you can browse into the former)
* Use link/append to navigate into the saved file with the assets. You should
see the preview in thumbnail mode.