Controls: Add support for OnRendered event

[Jemt]

Being able to determine when a control is rooted in DOM would be helpful.

[Jemt]

WSTreeView could also benefit from this - the Render() function makes sure data is loaded when rendered - but this doesn't work if GetDomElement() is used to obtain the DOM representation, and rooted "manually". So OnRendered should probably cover both Rendered and Rooted.