[FIX] web: qweb: correctly set preprocess_node on qweb engine

issue: xml template are not translated anymore

rev d56b35e2 moved qweb code inside the `qweb.js` file and also changed
the behavior with the introduction of a `QWeb` factory returning instances
of qweb rendering engine. However, we have to set on the rendering engine
instance a `preprocess_node` function that will translate the qweb
templates.

The solution for this issue (also introduced with rev d56b35e2) was
to set this function on the prototype of the QWeb factory, and it was
obviously not enough as it has to be attached to the rendering engine
instance.

In order to benefit from the fact that the `preprocess_node` function
is attached to the prototype of the QWeb factory, we could attach it to
the rendering engine instance with something like
`qweb.preprocessed_node = this.preprocessed_node`, however i find that
conceptually attaching a translate function to a factory does not make
sense, si i simply made `preprocess_node` a free function and i set it
on rendering engine instance.

addons/web/static/src/js/framework/qweb.js

@@ -16,10 +16,11 @@ function QWeb(debug, default_dict) {
         'csrf_token': odoo.csrf_token,
     });
     qweb.debug = debug;
+    qweb.preprocess_node = preprocess_node;
     return qweb;
 }
 
-QWeb.prototype.preprocess_node = function() {
+function preprocess_node() {
     // Note that 'this' is the Qweb Node
     switch (this.node.nodeType) {
         case Node.TEXT_NODE:
@@ -43,7 +44,7 @@ QWeb.prototype.preprocess_node = function() {
                 }
             }
     }
-};
+}
 
 return QWeb;