Implement a screenshot command for Message mode

Implement a screenshot command for Message mode

* doc/misc/message.texi (MIME): Document it.

* lisp/gnus/message.el (message-screenshot-command): New variable.
(message-mode-map): New keystroke and menu item.  Also add
mml-attach-file to the menu.
(message-insert-screenshot): New command.

* lisp/gnus/mml.el (mml-parse-1): Allow having
content-transfer-encoding already in the part, so that we can have
inline base64-encoded binaries in the Message buffers.