[IMP] tags: introduce xml tag

Big change! This commit introduces an xml function tag to easily define
inline templates.

This is a pretty big change toward single file owl components

Part of #284

src/router/Link.ts

@@ -1,18 +1,18 @@
 import { Component } from "../component/component";
+import { xml } from "../tags";
 import { Destination, RouterEnv } from "./Router";
 
-export const LINK_TEMPLATE_NAME = "__owl__-router-link";
-export const LINK_TEMPLATE = `
-    <a  t-att-class="{'router-link-active': isActive }"
-        t-att-href="href"
-        t-on-click="navigate">
-        <t t-slot="default"/>
-    </a>`;
-
 type Props = Destination;
 
 export class Link<Env extends RouterEnv> extends Component<Env, Props, {}> {
-  static template = LINK_TEMPLATE_NAME;
+  static template = xml`
+    <a  t-att-class="{'router-link-active': isActive }"
+        t-att-href="href"
+        t-on-click="navigate">
+        <t t-slot="default"/>
+    </a>
+  `;
+
   href: string = this.env.router.destToPath(this.props);
 
   async willUpdateProps(nextProps) {

src/router/RouteComponent.ts

@@ -1,15 +1,15 @@
 import { Component } from "../component/component";
+import { xml } from "../tags";
 
-export const ROUTE_COMPONENT_TEMPLATE_NAME = "__owl__-router-component";
-export const ROUTE_COMPONENT_TEMPLATE = `
+export class RouteComponent extends Component<any, {}, {}> {
+  static template = xml`
     <t t-foreach="routes" t-as="route">
         <t t-if="env.router.currentRouteName === route.name">
             <t t-component="{{route.component}}" t-props="env.router.currentParams"/>
         </t>
-    </t>`;
+    </t>
+  `;
 
-export class RouteComponent extends Component<any, {}, {}> {
-  static template = ROUTE_COMPONENT_TEMPLATE_NAME;
   routes: any[] = [];
   constructor(parent, props) {
     super(parent, props);

src/router/Router.ts

@@ -1,7 +1,5 @@
 import { Env } from "../component/component";
 import { QWeb } from "../qweb/index";
-import { ROUTE_COMPONENT_TEMPLATE, ROUTE_COMPONENT_TEMPLATE_NAME } from "./RouteComponent";
-import { LINK_TEMPLATE, LINK_TEMPLATE_NAME } from "./Link";
 import { shallowEqual } from "../utils";
 
 type NavigationGuard = (info: {
@@ -84,10 +82,6 @@ export class Router {
       this.routes[partialRoute.name] = partialRoute as Route;
       this.routeIds.push(partialRoute.name);
     }
-
-    // setup link and directive
-    env.qweb.addTemplate(LINK_TEMPLATE_NAME, LINK_TEMPLATE);
-    env.qweb.addTemplate(ROUTE_COMPONENT_TEMPLATE_NAME, ROUTE_COMPONENT_TEMPLATE);
   }
 
   //--------------------------------------------------------------------------