[ADD] base_section_subtotal: for Odoo 13.0

Adds to the section and note renderer to look for a new XML attr 'section-subtotal-field'.

base_section_subtotal/views/assets_backend.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<odoo>
+
+    <template id="assets_backend" name="assets.backend" inherit_id="web.assets_backend">
+        <xpath expr="//script[last()]" position="after">
+            <script type="text/javascript" src="/base_section_subtotal/static/src/js/widgets.js"></script>
+        </xpath>
+    </template>
+
+</odoo>