Add "Send messages through Outbox folder" option

Users ask from time to time for the old behaviour when the messages
used to be saved through Outbox, rather than the composer being opened
all the time the message is sending. The change is pretty simple,
thus why not to add it.

Note the Outbox is not flushed after sending, the users is responsible
to invoke the Outbox flush on his/her own. This also allows a user
to postpone the message send for later.