core: Add "source-tags" property

So that more information can be conveyed about the source's content
to the front-end applications, such as:
- the genre of the media (tv, cinema, radio, music, etc.)
- the countries the media is relevant to
- whether the media served is from the same machine, or from
  the local network

https://bugzilla.gnome.org/show_bug.cgi?id=724019

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
test-ui: Show "source-tags" property

https://bugzilla.gnome.org/show_bug.cgi?id=724019