WIP: Support subcomponents in radio groups

The main purpose of this MR is to add support for specifying radio inputs via gl-form-radio subcomponents as children of the gl-form-radio-group component.

Radio inputs can be defined either using the options prop of gl-form-radio-group, or using gl-form-radio child components, or a combination of the two. Radios defined by subcomponents in the default slot always appear last, after those defined by the options prop. The first slot can be used to make child gl-form-radio components appear before everything else.

Various other remarks:

• Move everything under the form_radio_group namespace, since this is really an implementation of radio groups rather than standalone radios.
• Restructure examples to illustrate options/subcomponents usage, and styling possibilities, for better consistency with other components.
• Allow consumers to opt out of the default plain styling, rather than mandating plain.
• Remove superfluous id and name attributes from examples.

This is blocked by #188 (closed).

The aim is to address #167 (closed) and #189 (closed).