Add a new message operation "New Message to Recipients"

This is equivalent to starting to compose a new message, but the
recipients (To, Cc, Bcc, Reply-To) are copied from the selected message.
Useful for starting a new conversation with the same people which will
not get grouped or threaded with the original.

The operation is available from the "Message" menus in the KMail main
window and the reader window.

Internally this adds a new parameter to ComposeMessageJob to specify the
original message.

GUI:
I18N: