simplify style system

luigi/widget/radio.lua

@@ -1,6 +1,17 @@
 --[[--
 A radio button.
 
+Radio buttons change their @{attribute.value|value} attribute to
+`true` when pressed. Radio buttons should also have a `group`
+attribute. When a radio button is pressed, other radio buttons
+in the same layout with the same `group` attribute change their values
+to `false`.
+
+Changing the value of a radio button causes it to change its appearance to
+indicate its value. The standard themes use the @{attribute.icon|icon}
+attribute for this purpose. If a custom icon is provided when using the
+standard themes, the widget's value should be indicated in some other way.
+
 @widget radio
 --]]--
 
@@ -24,10 +35,5 @@ return function (self)
         end
     end)
 
-    self:onChange(function ()
-        local subtype = self.value and 'radio.checked' or 'radio.unchecked'
-        self.type = { 'radio', subtype }
-    end)
-
     self.value = not not self.value
 end