Fix metatable inheritance rules

gfx/element.lua

@@ -18,6 +18,7 @@ local Element = {
   id = "",
   onClick = nil
 }
+Element.__index = Element
 
 local function createWindow(element)
   return window.create(element.render.parent, element:getX(), element:getY(), element:getWidth(), element:getHeight(), element:isVisible())
@@ -28,7 +29,7 @@ function Element:new(o)
   local obj = o or {}
 
   setmetatable(obj, self)
-  obj.__index = self
+  obj.__index = obj
 
   if obj.render and obj.render.parent then
     obj:_reload()