Fix/finish implementation of integer-based LUC types

Fix/finish implementation of integer-based LUC types

This commit fixes the use of integers as keys for GHashTable. It also
parses the prioritised LUC types into integer values and stores them
in a GArray. The code for computing the start order is updated to also
use integers for the startup groups.

This change also fixes luc_starter_compare_luc_types(), which (a) didn't
work at all and (b) had to be converted to comparing integers, not
strings.