Add support for groups of keys with identical names

Groups of keys with identical names can come from different sources,
e.g. gpg.conf and the group configuration of the application. Groups
of keys with identical names, but different ids, can come from the
group configuration of the application.

Make the key list models work with groups of keys with identical names
by looking up groups by source, id, and name.
Make the KeyCache work with groups of keys with identical names by
allowing multiple groups per name.

GnuPG-bug-id: 5175