[IMP] owl: add basic support for sub roots

In this commit, we extend the owl App class to support multiple sub
roots. This is useful for situations where we want to mount sub
components in non-managed DOM. This is exactly what the Knowledge app is
doing, with mounting views in an html editor.

Currently, this requires some difficult and fragile hacks, and still,
the result is that it is very easy to mix components from the main App
and a SubApp.  But Knowledge does not actually care about creating a sub
app. It only needs the possibility to mount sub components in dynamic
places.

closes #1640

doc/reference/app.md

@@ -6,6 +6,7 @@
 - [API](#api)
 - [Configuration](#configuration)
 - [`mount` helper](#mount-helper)
+- [Roots](#roots)
 - [Loading templates](#loading-templates)
 
 ## Overview
@@ -92,6 +93,33 @@ Most of the time, the `mount` helper is more convenient, but whenever one needs
 a reference to the actual Owl App, then using the `App` class directly is
 possible.
 
+## Roots
+
+An application can have multiple roots. It is sometimes useful to instantiate
+sub components in places that are not managed by Owl, such as an html editor
+with dynamic content (the Knowledge application in Odoo).
+
+To create a root, one can use the `createRoot` method, which takes two arguments:
+
+- **`Component`**: a component class (Root component of the app)
+- **`config (optional)`**: a config object that may contain a `props` object or a
+  `env` object.
+
+The `createRoot` method returns an object with a `mount` method (same API as
+the `App.mount` method), and a `destroy` method.
+
+```js
+const root = app.createRoot(MyComponent, { props: { someProps: true } });
+await root.mount(targetElement);
+
+// later
+root.destroy();
+```
+
+Note that, like with owl `App`, it is the responsibility of the code that created
+the root to properly destroy it (before it has been removed from the DOM!). Owl
+has no way of doing it itself.
+
 ## Loading templates
 
 Most applications will need to load templates whenever they start. Here is