This interface needs to implemented by any class that wants to be notified by
a GUI property when its state has changed.
It forces the implementation of the
updateGUI() method in order to
react on the changed GUI property.
Note that if a class acts as a notification target for multiple properties at
the same time, there might be unwanted side effects. In that case, it may be
advisable to compose the notification target class of smaller sub classes
that each act as a notification target for one of the properties.