[ADD] misc: component Portal

The component Portal is used to teleport the content
in its slot as a child of an element
somewhere else in the DOM, usually 'body'

It is designed to be transparent and is just here to do the teleportation task

OwlEvents (with method `Component.trigger`) are redirected from
any Component instanciated within the Portal (and therefore teleported
somewhere else) to the place it would have been without teleportation
i.e. they are re-triggered onto the Portal root node

Co-authored-by: Aaron Bohy <aab@odoo.com>

closes #184
closes #466

src/index.ts

@@ -12,6 +12,7 @@ import * as _store from "./store";
 import * as _utils from "./utils";
 import * as _tags from "./tags";
 import { AsyncRoot } from "./misc/async_root";
+import { Portal } from "./misc/portal";
 import * as _hooks from "./hooks";
 import * as _context from "./context";
 import { Link } from "./router/link";
@@ -29,7 +30,7 @@ export const router = { Router, RouteComponent, Link };
 export const Store = _store.Store;
 export const utils = _utils;
 export const tags = _tags;
-export const misc = { AsyncRoot };
+export const misc = { AsyncRoot, Portal };
 export const hooks = Object.assign({}, _hooks, {
   useContext: _context.useContext,
   useDispatch: _store.useDispatch,