[IMP] Allow app to be mounted in shadow DOM

This commit allow owl Apps to be mounted into a shadow DOM.

tests/shadow_dom/shadow_dom.test.ts

@@ -0,0 +1,67 @@
+import { App, Component, useRef, xml } from "../../src";
+import { status } from "../../src/runtime/status";
+import { makeTestFixture, snapshotEverything } from "../helpers";
+
+let fixture: HTMLElement;
+
+snapshotEverything();
+
+beforeEach(() => {
+  fixture = makeTestFixture();
+});
+
+describe("shadow_dom", () => {
+  test("can mount app", async () => {
+    class SomeComponent extends Component {
+      static template = xml`<div class="my-div"/>`;
+    }
+
+    const container = document.createElement("div");
+    fixture.appendChild(container);
+    const shadow = container.attachShadow({ mode: "open" });
+    const app = new App(SomeComponent);
+    const comp = await app.mount(shadow);
+    const div = shadow.querySelector(".my-div");
+    expect(div).not.toBe(null);
+    expect(shadow.contains(div)).toBe(true);
+    app.destroy();
+    expect(shadow.contains(div)).toBe(false);
+    expect(status(comp)).toBe("destroyed");
+  });
+
+  test("can bind event handler", async () => {
+    let a = 1;
+    class SomeComponent extends Component {
+      static template = xml`<button t-on-click="add">Click</button>`;
+
+      add() {
+        a = 3;
+      }
+    }
+    const container = document.createElement("div");
+    fixture.appendChild(container);
+    const shadow = container.attachShadow({ mode: "open" });
+    await new App(SomeComponent).mount(shadow);
+    expect(a).toBe(1);
+    shadow.querySelector("button")!.click();
+    expect(a).toBe(3);
+  });
+
+  test("useRef hook", async () => {
+    let comp: SomeComponent;
+    class SomeComponent extends Component {
+      static template = xml`<div t-ref="refName" class="my-div"/>`;
+      div = useRef("refName");
+      setup() {
+        comp = this;
+      }
+    }
+    const container = document.createElement("div");
+    fixture.appendChild(container);
+    const shadow = container.attachShadow({ mode: "open" });
+    const mountedProm = new App(SomeComponent).mount(shadow);
+    expect(comp!.div.el).toBe(null);
+    await mountedProm;
+    expect(comp!.div.el).toBe(shadow.querySelector(".my-div"));
+  });
+});