[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,

src/component/component.ts

@@ -111,21 +111,22 @@ export class Component<T extends Env, Props extends {}> {
    * hand.  Other components should be created automatically by the framework (with
    * the t-component directive in a template)
    */
-  constructor(parent?: Component<T, any>, props?: Props) {
+  constructor(parent?: Component<T, any> | null, props?: Props) {
     Component.current = this;
 
-    const id: number = nextId++;
     let constr = this.constructor as any;
+    const defaultProps = constr.defaultProps;
+    if (defaultProps) {
+      props = this.__applyDefaultProps(props, defaultProps);
+    }
+    this.props = <Props>props;
+    if (QWeb.dev) {
+      QWeb.utils.validateProps(constr, this.props);
+    }
+
+    const id: number = nextId++;
     let depth;
     if (parent) {
-      const defaultProps = constr.defaultProps;
-      if (defaultProps) {
-        props = this.__applyDefaultProps(props, defaultProps);
-      }
-      this.props = <Props>props;
-      if (QWeb.dev) {
-        QWeb.utils.validateProps(constr, this.props);
-      }
       this.env = parent.env;
       const __powl__ = parent.__owl__;
       __powl__.children[id] = this;
@@ -136,7 +137,6 @@ export class Component<T extends Env, Props extends {}> {
       if (!this.env.qweb) {
         this.env.qweb = new QWeb();
       }
-      this.props = (undefined as unknown) as Props;
       this.env.qweb.on("update", this, () => {
         if (this.__owl__.isMounted) {
           this.render(true);

tests/component/component.test.ts

@@ -71,9 +71,9 @@ class WidgetA extends Widget {
 //------------------------------------------------------------------------------
 
 describe("basic widget properties", () => {
-  test("props is not defined on root components", async () => {
-    const widget = new Widget();
-    expect(widget.props).toBe(undefined);
+  test("props is set on root components", async () => {
+    const widget = new Widget(null, {});
+    expect(widget.props).toEqual({});
   });
 
   test("has no el after creation", async () => {