[nemo-qml-plugin-email] Run next offline action when in offline mode and  current action is online.

When in offline mode, if current action requires network, we try to
run all offline action in the queue in the meantime.
I am not entirely convinced about this check. It probably works in the case where
a) the current action is an online action
b) the phone is in offline
c) there's an offline action in the queue

But what if there are no offline actions in the queue? You wil execute the next online action. And this could happen as many times as enqueue is called for a new online action. You would end up in N online actions started but blocked... This is at least the impression that I got. Not entirely sure if I got it right, though.