Listener to indicator deinitialization.
Indicator must register listener by IIndicatorsProvider.addIndicatorStopListener(IIndicator, IStopListener) method.
There is no specific moment when it is known that some indicator instance is not used anymore,
so onStop() method will be called when this instance became unreachable through strong reference (or on application shutdown).
Therefore, it is preferable to not implement IStopListener interface by indicator directly, but use for it static inner class.