auto activate / deactivate WFSPositionTracker when GUI is opened

When opening the WFSPositionTracker via the menu now it activates
immediately. If it wasn't already active before, closing the window
will also deactivate the tracker. If you want to use the tracker
without GUI make sure to activate it via code:
WFSPositionTracker.start; before opening the WFSPositionTrackerGUI.