add mouse wheel events and scroll attribute

luigi/attribute.lua

@@ -122,6 +122,20 @@ function Attribute.style (widget, value)
     widget.reshape(widget.parent or widget)
 end
 
+--[[--
+Scroll ability.
+
+Should contain `true` or `false` (or `nil`).
+
+If set to `true`, moving the scroll wheel over the widget will adjust
+its scroll position when the widget's contents overflow its boundary.
+
+@attrib scroll
+--]]--
+function Attribute.scroll (widget, value)
+    widget.attributes.scroll = value
+end
+
 --[[--
 Size Attributes.