Use separate XML files for views

view/tax_code_tree2.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0"?>
+<!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
+this repository contains the full copyright notices and license terms. -->
+<tree string="Tax Codes" keyword_open="1">
+    <field name="name"/>
+    <field name="code"/>
+    <field name="sum"/>
+</tree>