Always add data sources in SystemMonitorEngine::sourceRequestEvent()
Otherwise if we show the plasma-nm applet twice, m_sensors won't be empty
and we won't be able to fect the connection speed data.
but they don't really explain why setData() was called only if m_sensors
- plasmoidviewer -a org.kde.plasma.networkmanagement
- open the networkmanager applet twice
- make sure the connection's speed graph works the 2nd time.
Reviewers: Plasma, jgrulich
Differential Revision: https://phabricator.kde.org/D5973