[IMP] add default order parameter to list views

List views can now be sorted by default with a simple keyword 'order'.
For example

...
<field name="arch" type="xml">
    <tree string="Product Variants" order="name">
        <field name="name"/>
        ...

addons/web/static/src/js/view_list.js

@@ -354,6 +354,12 @@ instance.web.ListView = instance.web.View.extend( /** @lends instance.web.ListVi
             this.sidebar.$el.hide();
         }
         //Sort
+        var default_order = this.fields_view.arch.attrs.order,
+            unsorted = !this.dataset._sort.length;
+        if (unsorted && default_order) {
+            this.dataset.sort(default_order);
+        }
+
         if(this.dataset._sort.length){
             if(this.dataset._sort[0].indexOf('-') == -1){
                 this.$el.find('th[data-id=' + this.dataset._sort[0] + ']').addClass("sortdown");