Create abstract interface that will be implemented by MessageComposer

Composer: MessageComposer class implements the OutgoingMessage
interface. Submission gets a OutgoingMessage* though its ctor.

Let the MessageComposer class be accessed by members of Gui::ComposeWidget
and tests_Composer_Submission by their own copy of MessageComposer*.
WIP: Create Composer::RedirectMessageComposer

This class implemented the Composer::MessageData interface and
will be used for message redirection (or bouncing).