pls: Add new Grilo Playlist lib

This utility library allows both plugins and applications to read
playlists as if they were containers, using an API similar to the
Sources.

With additional fixes from Juan A. Suarez Romero and
Bastien Nocera.

https://bugzilla.gnome.org/show_bug.cgi?id=695303

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
pls: Include src/ in introspection

Needed to find Grl-0.2.gir

https://bugzilla.gnome.org/show_bug.cgi?id=695303
ui-test: Handle playlists in the "/" filesystem

https://bugzilla.gnome.org/show_bug.cgi?id=695303