Blender Git Loki

IMB_thumb: Allow blen-handling func to also load datablock previews from .blend file.

Notes:
* Not yet used, this is the last piece of 'side changes' before the big filebrowser commit.
* We can probably be more effective here (like e.g. reading and storing all previews for a given
group in thumbnails cache at once, instead of re-opening and parsing the whole file each time),
but will do this later.