Use aboutToQuit signal to sync KSharedConfig

This seems more robust then relying on the destruction of
the KleopatraApplication object. I have tested it by logging
out as a Windows user from a Windows session. And received the
aboutToQuit but the dtor of the KleopatraApplication was not
called or at least at this point the sync did not work anymore.

(cherry picked from commit b785819ea30199cc3c0463739f1066487d495306)

• GnuPG-Bug-Id: T6667