[REF] asyncroot: create asyncroot component

This component replaces the t-asyncroot directive.

src/misc/async_root.ts

@@ -0,0 +1,19 @@
+import { Component } from "../component/component";
+import { xml } from "../tags";
+
+/**
+ * AsyncRoot
+ *
+ * Owl is by default asynchronous, and the user interface will wait for all its
+ * subcomponents to be rendered before updating the DOM. This is most of the
+ * time what we want, but in some cases, it makes sense to "detach" a component
+ * from this coordination.  This is the goal of the AsyncRoot component.
+ */
+
+export class AsyncRoot extends Component<any, any> {
+  static template = xml`<t t-slot="default"/>`;
+
+  async __updateProps(nextProps, parentFiber) {
+    this.render(parentFiber.force);
+  }
+}