[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_esc.test.ts.snap

@@ -180,7 +180,7 @@ exports[`t-esc t-esc=0 is escaped 2`] = `
     ctx[isBoundary] = 1;
     let b2 = block2();
     ctx[zero] = b2;
-    let b3 = callTemplate_2(ctx, node, key + \`__1\`);
+    let b3 = callTemplate_2.call(this, ctx, node, key + \`__1\`);
     return block1([], [b3]);
   }
 }"