Builder can't be accessed when there is no selection.
RenderPanel is specific to JChemPaint. Decoupling this from the IRenderer interface means we should be more modular and able to utilises the implementations in CDK. Not sure if it's correct as accessible from IChemModelRelay but okay for now. Note the only usage was in the RGroupHandler.
A little cleaner to parse the main JCP panel to the RGroupHandler at creation. This removes the JCP specific panel from the IChemModelRelay.