KeyListSortFilterProxyModel: Consider key filters when checking whether groups match

Previoulsy, key groups were always accepted here (when no filter string was set).
This results in somewhat unexpected results in Kleopatra's CertificateSelectionDialog.
The new behavior checks that at least one key in the group matches the key filter.

(cherry picked from commit 8394c4c0336d61c05672661cc664db50d40bf86d)

• GnuPG-Bug-ID: T6970