These changes fix bug NB#78736.

These changes fix bug NB#78736.
* src/modest-mail-operation.[ch]:
        * New mail operation run_queue. This mail operation runs when
          the send queue is looping through messages. This is used to
          warrant that we don't kill modest while we're sending
          messages, in a safer way (we were trying to detect the mail
          send attempt result, and it was not warranted to happen).
        * Remove all the code to process sent and error signals in
          send queue to finish send mail operations.
        * Now both update_account and send_mail operations notify end
          after setting up the send queue, not after knowing the send
          queue has sent the message. This way we rely now on the
          autoconfigured mail operation that lives with the send queue
          runnings.
* src/modest-tny-send-queue.c:
        * Now we hook on queue-start signal. This is for creating a mail
          operation that lives while we're trying to send messages.

pmo-trunk-r4083