Support using HighchartJsExpr for $callback in Highchart::render()?

[C-Duv]

The $callback parameter of Highchart::render() is there to support the third parameter of the Highcharts.Chart() JavaScript method: callback ("Function to run when the chart has loaded and and all external images are loaded.").

This $callback parameter is defined as a string (as per the DocBlock): I think it would be useful to be able to pass HighchartJsExpr objects, just as we do for other chart options.