Blender Git Loki

Some cleanup...

Merge branch 'master' into ui-preview-buttons

Previews: use a ghash to store previews of every path.

Simpler, sanier, and avoids Icons to have to own ImagePreview data even!

Merge branch 'master' into ui-preview-buttons

ImagePreview: merging changed and user_edited into a single flag

Merge branch 'master' into ui-preview-buttons

Initial support for icon/preview/thumbnail of files, including py API.

All this is rather rough at the edges still, but seems to be working from quick tests.
Basically, it:
* Rework a bi BKE_icon, adding a special kind of icons, which type is 0, and which
own a PreviewImage in their obj field.
* Add helper to generate such icon from a file path.
* Add RNA API (to bpy.data currently).

So now, one can request a 'Preview' for a given path, that preview has an icon_id
that can be passed as 'icon_value' parameters of UILayout functions.

Main issue currently is, if you request several time same path, you'll get different
icons/previews each time, maybe we want to store paths in another ghash instead?

Initial import of part of work done on Preview stuff from asset-experiments branch.