Use HeaderBars for composers

Move the buttons from the bottom of the composer into the header.  The
draft save text is moved into the composer toolbar, for lack of a better
location.  When it's inline, we insert our own close button, so we can
control what it does.  When it's detached (and not using Unity), we
enable to default close button.

https://bugzilla.gnome.org/show_bug.cgi?id=731537