Add Start and End events to MouseWheelAdapter

Add onMouseWheelStart and onMouseWheelEnd to give some beginning/end timeframe to mouse adapter. The idea is that when a mousewheel first happens, a MouseWheelStart event is dispatched, followed by a MouseWheel, followed by a debounced MouseWheelEnd. When no mousewheel events are detected for 50ms, the MouseWheelEnd event is dispatched.