Added a test implementation for a new GaugeModel which should be more lightweight

This GaugeModel only fires one event if the model changes. The model does not contain JavaFX properties but only "old" JavaBeans style properties. Each property has it's own EventType so that you could attach one Listener to the GaugeModel and in the handle(GaugeEvent event) method you could react on each property change by checking for the EventType.