Display total count, not unread, in headerbar for Drafts/Outbox

This is a similar heuristic as in the folder list, as unread counts
in these folders is uninteresting.
--enable-unity flag to deal with GtkHeaderBar issues: Refs bgo#727624

Unity doesn't deal well with pure GtkHeaderBar apps, so this allows
for the GtkHeaderBar to be treated like a toolbar and the window
retain its chrome.