New metadata source proposal: local-metadata

[Guillaume Emont <gemont igalia com>]

Hi all,

I'm thinking of writing a new metadata source that would be there to
provide metadata that can be obtained locally, while not being provided
by the proper back end of any source.

The metadata that come in mind for now are:
 - thumbnails in .thumbnails/
 - album artwork in .cache/media-art
I'm open to more ideas.

Possible alternatives I see are:
 * include that code in all sources that could need it. The filesystem
source already provide .thumbnails/ thumbnails using GIO, and we could
have the tracker plugin do it in the same way. That means duplicated
code. Note that even in my solution, the .thumbnails code will be quite
similar to the stuff in filesystem, but removing it from filesystem
doesn't make much sense, as it's a one-liner in that context.
 * not get that metadata

Also, I see two ways of implementing that:
 * as a separate plugin
 * as a metadata source added to the filesystem plugin

Comments/ideas/better name to propose?

Guij