Dataform field radiobutton rendering does not apply the option separator.

The separator setting of the radiobutton field is stored in param3 but it is only the mode and for rendering purposes we need to retrieve the separator info. This is done in the selectmulti field by a method get_separator(), but such a method is missing from the radiobutton field. A simple solution is thus adding a method dataformfield_radiobutton_radiobutton::get_separator().