[IMP] owl: add a new mount method

doc/reference/utils.md

@@ -21,8 +21,8 @@ argument, it executes it as soon as the DOM ready (or directly).
 ```js
 Promise.all([loadFile("templates.xml"), owl.utils.whenReady()]).then(function ([templates]) {
   const qweb = new owl.QWeb({ templates });
-  const app = new App({ qweb });
-  app.mount(document.body);
+  const env = { qweb };
+  await mount(App, { env, target: document.body });
 });
 ```
 
@@ -31,8 +31,8 @@ or alternatively:
 ```js
 owl.utils.whenReady(function () {
   const qweb = new owl.QWeb();
-  const app = new App({ qweb });
-  app.mount(document.body);
+  const env = { qweb };
+  await mount(App, { env, target: document.body });
 });
 ```