Split over lines to diff more easily.
In preparation for it being removed, see: T47811
This can be kept on file level but best keep out of operator functions.
Requested by Aaron Carlisle (@blendify) over IRC.
Not much to add, you can now clear previews from current .blend file, or a set of non-opened files. Likewise, you can generate previews (for mat/tex, objects, groups, scenes, ...).