filesystem: Use new grl-pls library

The library now handles creating GrlMedia from GFileInfos for us.
We're also using the library to handle playlists files as containers
if the "handle-pls" configuration option is set to TRUE.

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

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>