Blender Git Loki

Second try to fix missing previews of mat/tex/etc. in .blend files.

This time, it's a dedicated operator user has to run before saving the file.

And it recursively check all IDs linked from each scene, therefore rendering
materials etc. previews using a scene they are used in.

Note the renderengine issue is not completely addressed this way
(existing code for icon previews seems to ignore completely other engines,
and IDs not linked anywhere (fake-user ones) will be rendered with current scene's engine
as fallback, also you can get a material linked to an hidden object in a scene, etc.).

Reviewers: sergey, campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D980