Default order in list/kanban views (task #8527)

Add an 'order' attribute to the xml description of list and kanban views.  If this attribute is set, the list/kanban view will be initially sorted accordingly.

Example:

        <record id="product_product_tree_view" model="ir.ui.view">
            <field name="name">product.product.tree</field>
            <field name="model">product.product</field>
            <field eval="7" name="priority"/>
            <field name="arch" type="xml">

                <tree string="Product Variants" order="name">
                    <field name="default_code"/>
                    <field name="name"/>
...

This list view will be sorted by default by the 'name' field.