[IMP] component: simplify constructor API

It now takes two arguments: parent (optional, only for non-root
components) and props (optional). In the case of the root component,
the env is taken from the config (config.defaultEnv). If it doesn't
exists, the default env is created on the fly.

Closes #306

src/component/component.ts

@@ -1,6 +1,7 @@
 import { Observer } from "../core/observer";
 import { CompiledTemplate, QWeb } from "../qweb/index";
 import { h, patch, VNode } from "../vdom/index";
+import { config } from "../config";
 import "./directive";
 import { Fiber } from "./fiber";
 import "./props_validation";
@@ -106,25 +107,14 @@ export class Component<T extends Env, Props extends {}> {
   /**
    * Creates an instance of Component.
    *
-   * The root component of a component tree needs an environment:
-   *
-   * ```javascript
-   *   const root = new RootComponent(env, props);
-   * ```
-   *
-   * Every other component simply needs a reference to its parent:
-   *
-   * ```javascript
-   *   const child = new SomeComponent(parent, props);
-   * ```
-   *
    * Note that most of the time, only the root component needs to be created by
    * hand.  Other components should be created automatically by the framework (with
    * the t-component directive in a template)
    */
-  constructor(parent: Component<T, any> | T, props?: Props) {
-    const defaultProps = (<any>this.constructor).defaultProps;
+  constructor(parent?: Component<T, any>, props?: Props) {
     Component.current = this;
+
+    const defaultProps = (<any>this.constructor).defaultProps;
     if (defaultProps) {
       props = this.__applyDefaultProps(props, defaultProps);
     }
@@ -136,17 +126,18 @@ export class Component<T extends Env, Props extends {}> {
     if (QWeb.dev) {
       QWeb.utils.validateProps(this.constructor, this.props);
     }
-    let id: number = nextId++;
+
+    const id: number = nextId++;
+
     let depth;
-    let p: Component<T, any> | null = null;
-    if (parent instanceof Component) {
-      p = parent;
+    if (parent) {
       this.env = parent.env;
       const __powl__ = parent.__owl__;
       __powl__.children[id] = this;
       depth = __powl__.depth + 1;
     } else {
-      this.env = parent;
+      // we are the root component
+      this.env = config.env as T;
       this.env.qweb.on("update", this, () => {
         if (this.__owl__.isMounted) {
           this.render(true);
@@ -162,6 +153,7 @@ export class Component<T extends Env, Props extends {}> {
       });
       depth = 0;
     }
+
     const qweb = this.env.qweb;
 
     this.__owl__ = {
@@ -171,7 +163,7 @@ export class Component<T extends Env, Props extends {}> {
       pvnode: null,
       isMounted: false,
       isDestroyed: false,
-      parent: p,
+      parent: parent || null,
       children: {},
       cmap: {},
       currentFiber: null,