server: Port media-art lookup to libmediaart

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=710387
server,media-export: Use libmediaart for storing

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=710387
media-export: Also extract sidecar media-art

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=710387
engine-gst: Prevent critical after streaming

Signed-off-by: Jens Georg <mail@jensge.org>