GpgOL 1.3.0 replaces the body of a mail / decrypts attachments at runtime in the
read event. This is ok because it also catches write events and encrypts
attachments / wipes the mail body again in those events.

But in case GpgOL is deactivated through Outlook options we no longer see the
write event and so we don't wipe the mail. Leaking plaintext.

The workaround for that issue is clear "Don't deactivate GpgOL" but we can fix
this if we keep track of all Mailitems that need to be wiped and wipe them when
our Addin instance is released / deleted.