Propper disconnection of tracking device.

Set TrackingDeviceSource = nullptr at the end of disconnect.
The nullptr fixes the issue with connect/discon/connect -> crash.

Also, remove m_TrackingDeviceSource from Toolboxview. This variable
should only be handed by the worker class (threads...).

For Polhemus, Disconnect-signal is send to hardware.

Also... replace tab with spaces. Code style.