[FIX] qweb: add support for t-att-value on <select> tags

Before this commit, it was not possible to set the value of a select tag
by using the t-att-value attribute.

Doing so is not actually trivial because of the way the vdom works: it
processes the node attributes before its children are created, which
means that the vdom code tries to set the initial value of the select
before its children are created, which means that it is ignored. To make
it work, I added a node create hook which is called after the children
are completely processed.

closes #873

tests/qweb/qweb.test.ts

@@ -1950,6 +1950,30 @@ describe("special cases for some specific html attributes/properties", () => {
     patch(vnode2, vnode3);
     expect(elm.value).toBe("potato");
   });
+
+  test("select with t-att-value", () => {
+    const template = `
+      <select t-att-value="value">
+        <option value="potato">Potato</option>
+        <option value="tomato">Tomato</option>
+        <option value="onion">Onion</option>
+      </select>`;
+    qweb.addTemplate("test", template);
+    const vnode1 = qweb.render("test", { value: "tomato" });
+    const vnode2 = patch(document.createElement("select"), vnode1);
+    let elm = vnode2.elm as HTMLSelectElement;
+    expect(elm.value).toBe("tomato");
+
+    elm.value = "potato";
+    expect(elm.value).toBe("potato");
+
+    // rerender with a different value, and patch actual dom, to check that
+    // select value was properly reset by owl
+    const vnode3 = qweb.render("test", { value: "onion" });
+    patch(vnode2, vnode3);
+    expect(elm.value).toBe("onion");
+    expect(qweb.templates.test.fn.toString()).toMatchSnapshot();
+  });
 });
 
 describe("whitespace handling", () => {