Timeline Lazy loading of events

Timeline supports lazy loading of events during moving / zooming in the timeline. This makes sense when event's loading is time-consuming. Events are loaded lazy when p:ajax with event="lazyload" is attached to the p:timeline tag. The event class TimelineLazyLoadEvent contains one or two time ranges the events should be loaded for (two times ranges occur when you zoom out the timeline).

Note: The "lazyload" listener is not invoked again when the visible time range (incl. some hidden ranges defined by preloadFactor) already has lazy loaded events.