[IMP] point_of_sale: don't explicitly call order_changes

Instead watch for changes on paymentlines and update based on
that. This way it's no longer the developer's responsibility to
manually update the UI, it'll be taken of for him (the main reason for
using BackboneJS).

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

@@ -1900,7 +1900,6 @@ var PaymentScreenWidget = ScreenWidget.extend({
                 }
 
                 paymentline.set_amount(amount);
-                this.order_changes();
                 this.render_paymentlines();
                 this.$('.paymentline.selected .edit').text(this.format_currency_no_symbol(amount));
             }
@@ -2094,7 +2093,6 @@ var PaymentScreenWidget = ScreenWidget.extend({
             }
 
             this.render_paymentlines();
-            this.order_changes();
         }
     },
     render_paymentmethods: function() {
@@ -2130,7 +2128,6 @@ var PaymentScreenWidget = ScreenWidget.extend({
             'value': self.format_currency_no_symbol(value),
             'confirm': function(value) {
                 order.set_tip(field_utils.parse.float(value));
-                self.order_changes();
                 self.render_paymentlines();
             }
         });
@@ -2185,7 +2182,6 @@ var PaymentScreenWidget = ScreenWidget.extend({
         this.pos.get_order().clean_empty_paymentlines();
         this.reset_input();
         this.render_paymentlines();
-        this.order_changes();
         // that one comes from BarcodeEvents
         $('body').keypress(this.keyboard_handler);
         // that one comes from the pos, but we prefer to cover all the basis
@@ -2216,6 +2212,7 @@ var PaymentScreenWidget = ScreenWidget.extend({
         order.bind('all',function(){
             self.order_changes();
         });
+        order.paymentlines.bind('all', self.order_changes.bind(self));
         this.old_order = order;
     },
     // called when the order is changed, used to show if