Extend mail class for better reply/forward handling
* src/mail.cpp, src/mail.h (Mail::set_is_reply_crypto_mail), (Maill::is_reply_crypto_mail, m_is_reply_crypto_mail): New flag for replies. (Mail::remove_all_attachements): New helper. (Mail::is_block_html): Expose the flag.
This helps with:
- GnuPG-Bug-Id: T3986