Allow for easier subclassing of radio-elements that requires the same accessory

[Alxandr]

I recently needed to subclass the RadioElement class to add an image to the left of the text. There were probably other ways I could have done this, but I created a subclass of UITableViewCell meaning I couldn't simply do base.GetCell(), so I had to recreate the code that does the adding of the cell accessory. This turned out to be quite painful, because a lot of the needed objects are marked as internal, thus I edited the RadioElement class to easily allow for such scenarios.

[migueldeicaza]

Alxandr,

Would it be possible to update the samples to show this in action?