[IMP] qweb: re-add support of t-on directive

We add some test for the t-on directive.

For making them pass, it was necessary to change the code produced by
compileTForeach: the const declaration is not done by using generateId
and there was some conflict with the variable names produced in
captureExpression. Consequently, many snapshots had to be changed.

Code prettification has been done too.

tests/components/lifecycle.test.ts

@@ -919,7 +919,9 @@ describe("lifecycle hooks", () => {
       static template = xml`<div/>`;
       setup() {
         created = true;
-        onMounted(() => { mounted = true; })
+        onMounted(() => {
+          mounted = true;
+        });
       }
     }
     class Parent extends Component {