CiviMail draft (unsent) mailings should have Edit option rather than Resend

If you are half-way through preparing a CiviMail mailing when you stop, your mailing will appear in the mailings list but to continue editing it you have to "Resend" that mailing which creates a new copy. The Resend "button" should be called Edit when the mailing hasn't yet been sent, and clicking on Edit should enable you to edit and then send that mailing, rather than create a new mailing based on the earlier unsent mailing.