[IMP] component: add props on root components

This is a partial revert of commit 7d249d6f09.

The reason is that this changes made it much harder to unit test
components.  Before, we could simply instantiate a component
like this:
const my|Comp = new MyComponent(null, props);

and then simply test it.  This commit reestablish that possibility.

doc/reference/component.md

@@ -443,10 +443,14 @@ const app = new App();
 app.mount(document.body);
 ```
 
-The root component does not have a parent nor props. It will be setup with an
+The root component does not have a parent nor `props` (see note below). It will be setup with an
 [environment](environment.md) (either the `env` defined on its class, or a
 default empty environment).
 
+Note: a root component can however be given a `props` object in its constructor,
+like this: `new App(null, {some: 'object'});`.  It will not be a true `props`
+object, managed by Owl (so, for example, it will never be updated).
+
 ### Composition
 
 The example above shows a QWeb template with a sub component. In a template,