[IMP] qweb: turn handlers into function expressions only

For the sake of consistency with vanilla JS, and to allow some things
that were previously not possible.

tests/qweb/__snapshots__/t_ref.test.ts.snap

@@ -43,7 +43,7 @@ exports[`t-ref ref in a t-call 1`] = `
   let block1 = createBlock(\`<div><block-child-0/></div>\`);
   
   return function template(ctx, node, key = \\"\\") {
-    let b2 = callTemplate_2(ctx, node, key + \`__1\`);
+    let b2 = callTemplate_2.call(this, ctx, node, key + \`__1\`);
     return block1([], [b2]);
   }
 }"