Fixes NB#78602

Fixes NB#78602
   * modest-mail-operation-queue.[hc]:
      (modest_mail_operation_queue_get_by_source): returns the mail operations
         in the queue associated with a given source
   * widgets/modest-window-mgr.c:
      (modest_window_mgr_unregister_window): if the window that is being
         closed is a ModestMsgViewWindow, it cancels "open" and "receive"
         operations that have it as their source
   * maemo/modest-main-window.c:
      (on_mail_operation_started): only goes to transfer mode if it is the
         source of the operation whose start has been notified
   * maemo/modest-msg-view-window.c:
      (on_mail_operation_started): only goes to transfer mode if it is the
         source of the operation whose start has been notified
      (view_msg_cb): added checks for cancelled operations and empty fields

pmo-trunk-r3917