[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/config.ts

@@ -1,17 +1,20 @@
 import { QWeb } from "./qweb/index";
+import { Env } from "./component/component";
 
 /**
  * This file creates and exports the OWL 'config' object, with keys:
  *  - 'mode': 'prod' or 'dev',
+ *  - 'env': the environment to use in root components.
  */
 
 interface Config {
+  env: Env;
   mode: string;
 }
 
-const _config:Partial<Config> = {};
+export const config = {} as Config;
 
-Object.defineProperty(_config, "mode", {
+Object.defineProperty(config, "mode", {
   get() {
     return QWeb.dev ? "dev" : "prod";
   },
@@ -28,4 +31,18 @@ Object.defineProperty(_config, "mode", {
   },
 });
 
-export const config:Config = _config as Config;
+let env:Env;
+Object.defineProperty(config, "env", {
+  get() {
+    if (!env) {
+      env = {} as Env;
+    }
+    if (!env.qweb) {
+      env.qweb = new QWeb();
+    }
+    return env;
+  },
+  set(newEnv: Env) {
+    env = newEnv;
+  },
+});