Ensure that key filters read from config are sorted correctly

The config group names returned by KConfig::groupList() are in arbitrary
order, but we want them ordered by their number so that for example the
colors of the key filters are applied in the intended (deterministic)
order. We ensure this order by putting them into a map with the group
numbers as key.

To avoid duplicate code for loading the key filter config groups in
Kleopatra's Appearance settings we provide a function for this.

(cherry picked from commit d61d23ce2ab3d2e8fe578c60c9c56149bf9f6668)

• GnuPG-bug-id: T8279