[FIX] sale_coupon: sdd sequence to the list view

The coupon program applies only one rule based on the sequence.

Steps to reproduce the bug:

Create a promotion with 5% if untaxed amount is greater than 1,000$
Create a promotion with 10% if untaxed amount is greater than 2,000$
Create a promotion with 15% if untaxed amount is greater than 3,000$
Create a SO for 3500$
Click on "Update Promotions"

Bug:

The 5% promotion is applied because it is the first one in the list.

Solution:

Display the sequence field in the list view with a handle to sort the different
promotions.

closes odoo/enterprise#3209

views/sale_coupon_program_views.xml

@@ -116,6 +116,7 @@
         <field name="model">sale.coupon.program</field>
         <field name="arch" type="xml">
             <tree>
+                <field name="sequence" widget="handle"/>
                 <field name="name"/>
                 <field name="active"/>
             </tree>