metadata: Use queues instead of lists

Some of the lists used by the metabuilder have items appended to them. Appending
to a list is done in linear time and this highly affects efficiency. In order to
fix this, use GQueue which supports appending in constant time.

Modified by Ondrej Holy	<oholy@redhat.com>.

https://bugzilla.gnome.org/show_bug.cgi?id=757747