simplify style system

luigi/widget/check.lua

@@ -1,6 +1,14 @@
 --[[--
 A check box.
 
+Check boxes toggle their @{attribute.value|value} attribute between
+`true` and `false` when pressed.
+
+Changing the value of a check box 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 check
 --]]--
 
@@ -9,10 +17,5 @@ return function (self)
         self.value = not self.value
     end)
 
-    self:onChange(function ()
-        local subtype = self.value and 'check.checked' or 'check.unchecked'
-        self.type = { 'check', subtype }
-    end)
-
     self.value = not not self.value
 end