Do not show the audit log if dialog is closed with Esc

To fix the problem we switch the two buttons, i.e. we use the Yes button
for "Show Audit Log" and the No button for "Ok". Additionally, we set the
Dangerous option to make the No/Ok button the default button.

Also:
* Add support for window modality.
* Use KMessageBox::messageBox() if we don't have an Audit Log.

GnuPG-bug-id: 6268