Blender Git Loki

Move handling of ID blocks previews in thumbnail handler.

this is mandatory, since we do not keep previews for all listed items anymore
(with 10k datablocks would start to take too much mem). So we now generate (read)
them from thumbnailing threaded task as any other image, has several advantages:
* Quicker initial listing of data from .blend files.
* Since thumbnails are cached on disk, re-generating them is rather quick.
* General consistency!

Note thumbnail generation of datablocks could (should!) be enhanced, right now
it reads all previews for a given datatype in a file each time it has to generate
a single datablock thumbnail, we should generate all of those previews in a single
run. But this means handling cache dirty things (TBH_FAIL, outdating, etc.) on another
level so need some more work.

Also, still some memleaks issues with thumbnails, have to track those dose!