[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.
Note the 'm_enqueing' var, actions are only executed when we finish enqueing. This code does not check here for online or offline, it just checks if we are not enqueing anymore, if current action is null or current action is not running. When action are running this is not invoked, and next action will be assigned inside 'activityChanged' slot.

If there are no offline actions in the queue no element is moved, getNext just return first action in the queue like before, and then inside 'executeCurrent()' nothing happens if in offline mode.