modules/menusystem: add an arbitrary order variable to widgets

gamecore/modules/menusystem/widgets/init.lua

@@ -42,6 +42,8 @@ function BaseWidget:new(menu)
   self.canvas = {}
   self.canvas.texture = nil
   self.canvas.needRedraw = true
+
+  self.order = 0
 end
 
 function BaseWidget:register()