Add bead to support infinite scrolling and dynamic loading of content

[Harbs]

Added InfiniteVScroller in 4628588

The bead can be added to any component which has vertical scrolling. When the scrolling reaches the bottom of the component, the bead dispatches a scrollEnd event which can be handled to add content to the dataProvider or similar.

The bead has two properties: offset and interval. offset allows preemptive requesting content at a specific number of pixels before the bottom of the scroll is reached. interval specifies the minimum number of milliseconds between scrollEnd events to prevent rapid-fire requests.