[FIX] point_of_sale: scroll to bottom after weighing product

When switching back from the scale screen to the
product screen, the order summary was re-drawn
and always scrolled at the top.

This happens because the redraw takes place in the
background while the scale screen is still displayed,
so the new area does not know its size yet and
cannot be scrolled properly.

Swapping these operations around fixes the issue
and it happens fast enough to be invisible to the
user.

addons/point_of_sale/static/src/js/screens.js

@@ -272,8 +272,9 @@ var ScaleScreenWidget = ScreenWidget.extend({
         });
 
         this.$('.next,.buy-product').click(function(){
-            self.order_product();
             self.gui.show_screen(self.next_screen);
+            // add product *after* switching screen to scroll properly
+            self.order_product();
         });
 
         queue.schedule(function(){