Add quick print context menu entry

* src/gpgoladdin.cpp: Extend ribbon UI and callbacks.
* src/ribbon-callbacks.cpp (print_decrypted, get_is_crypto_mail):
New.

As the Quick Print and File -> Print do not work if mails
are only viewed in the Explorer this is a shortcut to
avoid that users have to explicitly open the mail before
printing.

• GnuPG-Bug-Id: T3762