Clean reply / forwards of unsigned S/MIME Mails

* src/mailitem-events.cpp (EVENT_SINK_INVOKE): Improve
reply / forward handling and check for HTML Blocked Mails to
clean them.
* src/windowmessages.cpp, src/windowmessages.h
(do_in_ui_thread_async): Extend to take delay as opt. param.
(CLEAR_REPLY_FORWARD): New message.
(gpgol_window_proc): Handle it.

This is ugly ugly ugly. As we don't have a reliable
way to detect when a reply / forward is filled with data
we just wait a second before wiping the reply if necessary.

The change in the write event to not invalidate the
last mail also poses a regression risk.

• GnuPG-Bug-Id: T3986