Fix sorting of key filter config groups by specificity

Key filters with a custom ID were not sorted correctly because the
comparison helper greaterThanBySpecificity tried to get those key filters
by ID using the config group name (which failed).

To fix the sorting we build a list of pairs (specificity, config group)
to sort the key filters by specificity. In particular, we use the custom
ID instead of the group name to get the key filters if a custom ID is
specified for a key filter.

• GnuPG-bug-id: T8079