A proper class for a frame containing radio buttons. This object has a
"value" property and will therefore be very easy to use in various API
binding a config property to a widget property.

A GimpIntRadioFrame is also what gimp_prop_int_radio_frame_new() will
return now.

gimp_prop_int_radio_box_new() on the other hand is being removed. It is
used nowhere and is unneeded. If someone really needs a non-labelled
group of radio buttons, they can also create a GimpIntRadioFrame, remove
the label and hide the borders. At least they will still be able to
easily bind it to a config property.