Change .is_stopped() to .isStopped()
Add .addEventTypes method to Lab Event Dispatcher (+ tests)

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It can be used to dynamically add new event types. D3.dispatch accepts
event types only as constructor arguments.
Add .mixInto(target) method to dispatch, rename it to DispatchSupport

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Implement PlaybackSupport mix-in ("steppableModel")

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Use new PlaybackSupport and DispatchSupport in LabModelerMixin

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Update Energy2D and Signal Generator modelers after LabModelerMixin changes

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