Added support for radio buttons in selectField.

Right now this only covers selectField when invoked with
SelectableOptions, but it's only specs and a small amount of
glue code missing for tuple lists and regular lists.

Radio buttons are created by passing an
asRadioButtons = true parameter to selectField. That
parameter repeates the element selected by the passed
selector, binding a label and radio button within once for
each of the provided options.
Added radio button support for selectField with tuples.
Added radio button support for selectField with simple objects.
Fix radio button specs to all bind to li.

This instead of the ul. It doesn't make a material difference
to the specs, but is more correct, since the previous version
repeated the whole ul instead of just repeating the li.