Blender Git Loki

Ensure previews are up to date when saving .blend file.

Previuosly, we would only write existing data - and big previews were
nearly never generated by default.

Now, if saving previews is enabled, `wm_file_write()` will ensure
all 'common' previewable datablocks do have up-to-date previews
(brushes remain TODO here for now, they have both a preview **and**
an icon_imbuf, sigh...).

This implied adding an 'immediate' non-job-background version of
`ED_preview_icon_job()`.

Note this does not include objects/groups/etc. It's not simple to generate
previews for those, will probably go for a user-launched py operator here...

Also, cleanup some debug prints!