Handle "send outgoing messages" properly for combined Outbox

* The "send outgoing messages" button doesn't work in the combined
  inbox (the case wasn't handled)
* Add code to loop through accounts, calling the Controller for each
  in this case
* Fixes (partially or completely) #2274389

Change-Id: I94e984247d43f93a4d6546b8c10f6ce149b091be
1 file changed