[FIX] web: qweb old inheritance mechanism

Before this commit, defining a extension of a template by using the
combination of directives 't-name' and 't-extend' ignored potential
extensions that could have been defined beforehand.

For instance, let's assume the following templates.
```xml
<t t-name="a">
    <div><span>1</span></div>
</t>
<t t-extend="a">
    <t t-jquery="span" t-operation="replace">
        <span>2</span>
    </t>
</t>
<t t-name="b" t-extend="a">
    <t t-jquery="div" t-operation="append">
        <span>b</span>
    </t>
</t>
```
Rendering template "b" displayed "1b" whereas we would expect "2b".

Moreover, when the extended template is itself an extension of
another template:
```xml
<t t-name="a">
    <div><span>a</span></div>
</t>
<t t-name="b" t-extend="a">
    <t t-jquery="div" t-operation="append">
        <span>b</span>
    </t>
</t>
<t t-name="c" t-extend="b">
    <t t-jquery="div" t-operation="append">
        <span>c</span>
    </t>
</t>
```
Rendering template "a" displayed "a", template "b" displayed "ab",
but template "c" displayed "ac", whereas we would expect "abc".

With this commit, other extensions done to a template are kept when
a new extension is defined. It relies on the templates order, and
takes into account all extensions that have *already* been defined.
This is exactly how the new qweb inheritance mechanism (using
't-inherit' and 't-inherit-mode') behaves.

closes odoo/odoo#54315

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>

addons/web/static/lib/qweb/qweb-test-extend.xml

@@ -37,7 +37,26 @@
         <t t-jquery="ul" t-operation="append"><li>[[cloned template]]</li></t>
     </t>
     <result id="jquery-extend-clone"><![CDATA[
-    <ul><li>one</li><li>[[cloned template]]</li></ul>
+    <hr/><ul class="main" title="Main Title" name="main-ul"><li>1</li><li>2</li><li>3</li><li>[[cloned template]]</li></ul><footer><b>[[end]]</b></footer>
 ]]></result>
 
+
+    <t t-name="a">
+        <div><span>Hi</span></div>
+    </t>
+    <t t-name="b" t-extend="a">
+        <t t-jquery="span" t-operation="after"><i>World</i></t>
+    </t>
+    <t t-name="c" t-extend="b">
+        <t t-jquery="span" t-operation="replace"><span>Hello</span></t>
+    </t>
+    <result id="a"><![CDATA[
+    <div><span>Hi</span></div>
+]]></result>
+    <result id="b"><![CDATA[
+    <div><span>Hi</span><i>World</i></div>
+]]></result>
+    <result id="c"><![CDATA[
+    <div><span>Hello</span><i>World</i></div>
+]]></result>
 </templates>

addons/web/static/lib/qweb/qweb2.js

@@ -268,6 +268,7 @@ QWeb2.Engine = (function() {
                             return this.tools.exception("Can't clone undefined template " + extend);
                         }
                         this.templates[name] = this.templates[extend].cloneNode(true);
+                        this.extend_templates[name] = (this.extend_templates[extend] || []).slice();
                         extend = name;
                         name = undefined;
                     }