Fix saving of certificate appearance aspects differing from default

Instead of checking for valid QVariant values or for set bool values to
decide whether to write a appearance config entry we compare the values
chosen by the user with the values in the system config (i.e. the one
shipped with libkleo) and write values that differ from the shipped
defaults.

(cherry picked from commit b80c58e973ed4f7c8974463437f7b9ed953adf4f)

• GnuPG-bug-id: T8258