[grilo-plugins] Add support for set_metadata in tracker plugin

[lionel g landwerlin linux intel com]

Hi all,

here is a small serie of patches to enable add set_metadata support to
tracker plugin. Currently we only save a few properties, which are
likely to be used in a UI to remember the last position, play count
and last play time of media items.

Regards,

--
Lionel Landwerlin