Rework custom about data handling

This uses Kleo::gpgvVerify instead of all the
ifdef Windows code to verify a settings file and
update the about data accordingly. Since the verified
settings file may contain additional settings it is stored
in the KleopatraApplication object for future use.

(cherry picked from commit f1c7d1ea0cc53bbce4d4e35e4408b3add52a1dd2)

• GnuPG-Bug-Id: T6797