[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/component/fiber.ts

@@ -222,6 +222,7 @@ export class Fiber {
     if (this.target) {
       this.target.appendChild(this.component.el!);
       inDOM = document.body.contains(this.target);
+      this.component.env.qweb.trigger("dom-appended");
     }
 
     // call patched/mounted hook on each fiber of (reversed) patchQueue