Add junk mail handling

* src/categorymanager.cpp (getJunkMailCategory): New.
* src/mail.cpp (Mail::decryptVerify_o): Handle junk.
* src/mail.h (Mail::m_is_junk): New.

Junk mails are special. An addon can't access the attachments
so we can't decrypt PGP-MIME. We just add a marker to let
the user now and register our folder event handler so
that the mail can be moved out of junk.

• GnuPG-Bug-Id: T4188