pixbuf-thumbnailer: Use a mime-type-specific loader when possible

Fixes #20 (closed), #32 (closed), #36 (closed), obsoletes !8 (closed).

This allows to fully use the capabilities of the specific pixbuf loader for the considered mime type according to the installed optional libraries. This is especially necessary to load some SVG or HEIF thumbnails.

On the other hand, if this specific loader doesn't work properly, it may be even more noticeable. This is the case for libopenraw, for which a dedicated plugin using another API has fortunately been implemented. So we just have to prioritize it in tumbler.rc.