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

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.