[IMP] pos_discount: usability improvements regarding discount setup

Before this, setting up the discount on a pos_config was a bit
confusing. Although iface_discount was enabled by default on the
pos_config, the discount button would not show up in the actual POS. The
reason for this is that the discount button only shows up when a
discount product has been set, and there is no default for that.

So in order to make things a bit clearer the following has been done:
- Group discount related options together
- Only show discount product and default percentage field when using discount
- Rename 'Discount percentage' -> 'Default percentage'
- Default iface_discount to False (because a discount_product_id has to
  be manually set anyway)
- Make discount_product_id required when discount is enabled
- Only allow discount_product_id candidates that have available_in_pos set,
  because the POS frontend only loads those products

addons/pos_discount/discount.py

@@ -13,10 +13,10 @@ class pos_config(osv.osv):
     _inherit = 'pos.config' 
     _columns = {
         'iface_discount': fields.boolean('Order Discounts', help='Allow the cashier to give discounts on the whole order.'),
-        'discount_pc': fields.float('Discount Percentage', help='The default discount percentage'),
-        'discount_product_id': fields.many2one('product.product','Discount Product', help='The product used to model the discount'),
+        'discount_pc': fields.float('Default Percentage', help='The default discount percentage'),
+        'discount_product_id': fields.many2one('product.product','Discount Product', domain="[('available_in_pos', '=', True)]",
+                                               help='The product used to model the discount'),
     }
     _defaults = {
-        'iface_discount': True,
         'discount_pc': 10,
     }

addons/pos_discount/views/views.xml

@@ -7,13 +7,13 @@
             <field name="model">pos.config</field>
             <field name="inherit_id" ref="point_of_sale.view_pos_config_form" />
             <field name="arch" type="xml">
-                <xpath expr="//group[@name='features']/group[1]" position="inside">
-                        <field name='iface_discount' />
-                </xpath>
                 <xpath expr="//group[@name='receipt']" position="after">
-                    <group string="Discounts" col="4" >
+                    <group string="Discounts">
+                        <field name='iface_discount' />
+                    </group>
+                    <group col="4" attrs="{'invisible': [('iface_discount', '=', False)]}">
                         <field name='discount_pc' />
-                        <field name="discount_product_id" />
+                        <field name='discount_product_id' attrs="{'required': [('iface_discount', '=', True)]}" />
                     </group>
                 </xpath>
             </field>