'Send' and 'Save Draft' are now asynchronous and run outside of
Evolution's MailMsg infrastructure.

Add an EActivityBar to the composer window so these asynchronous
operations can be tracked and cancelled even in the absense of a main
window.  Also add an EAlertBar to the composer window so error messages
can be shown directly in the window.

Instead of calling e_alert_dialog_run_for_args(), call e_alert_submit()
and pass the EMsgComposer as the widget argument.  The EMsgComposer will
decide whether to show an EAlertDialog or use the EAlertBar, depending
on the GtkMessageType of the alert.