metadata: Use sequences instead of lists

The metabuilder stores files and data in sorted lists. An insertion into a sorted
list is done in linear time, which highly affects efficiency. In order to fix
this, use GSequence instead which does insertion and deletion in logarithmic
time.

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

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