New local metadata source

[Guillaume Emont <gemont igalia com>]

These patches introduce a new metadata source that gets its data from the local
system. For now, it only get thumbnails for images and videos, through GIO's
G_FILE_ATTRIBUTE_THUMBNAIL_PATH (which gets existing thumbnails in
~/.thumbnails).

It could be expanded in the future, in particular we could add support for
http://live.gnome.org/MediaArtStorageSpec.

I've added two small tests, since core seems to have bugs around the automatic
use of metadata sources, therefore you can't really try this through
grilo-test-ui for instance.

Enjoy :)

Guij