[Accessibility, Windows] Implement HighContrast Mode Detection

When Alt + left Shift + Print Screed is pressed, Windows toggles between high contrast mode and regular mode. By default, JavaFX should change themes when this happens. Further, if an application provides its own theme, it will need to know when it should be applied.

A boolean property sounds like the natural way to implement this. That way, on platforms where the value can change, applications can be notified. On platforms that do not have this mode (Mac, Linux), we might want to allow the programmer to set the property so that application code just needs to listen.