Add radio buttons

[JD557]

Adds a new radioButton component.

Also adds some new methods to Ref so that radio button groups are easier to define with local mutability, for example, like this:

val nextValue = currentValue.asRef { valueRef =>
  radioButton(id = "0", area = column(0), buttonIndex = 0, label = "0")(valueRef)
  radioButton(id = "1", area = column(1), buttonIndex = 1, label = "1")(valueRef)
  radioButton(id = "2", area = column(2), buttonIndex = 2, label = "2")(valueRef)
}

As opposed to

val nextValue = {
  val valueRef= Ref(currentValue)
  radioButton(id = "0", area = column(0), buttonIndex = 0, label = "0")(valueRef)
  radioButton(id = "1", area = column(1), buttonIndex = 1, label = "1")(valueRef)
  radioButton(id = "2", area = column(2), buttonIndex = 2, label = "2")(valueRef)
  valueRef.value
}

or

val nextValue = {
  var _nextValue = currentValue
  _nextValue = radioButton(id = "0", area = column(0), buttonIndex = 0, label = "0")(_nextValue)
  _nextValue = radioButton(id = "1", area = column(1), buttonIndex = 1, label = "1")(_nextValue)
  _nextValue = radioButton(id = "2", area = column(2), buttonIndex = 2, label = "2")(_nextValue)
  _nextValue
}