[IMP] app: mount app in "first-child" position

We reintroduce the possibility to mount the app in first position in
a target. The option "self" has been dropped since it is now possible
for a component to have several top level nodes.

src/app.ts

@@ -1,5 +1,6 @@
 import { Component } from "./component/component";
 import { ComponentNode } from "./component/component_node";
+import { MountOptions } from "./component/fibers";
 import { Scheduler } from "./component/scheduler";
 import { TemplateSet } from "./qweb/template_helpers";
 
@@ -47,7 +48,7 @@ export class App<T extends typeof Component = any> extends TemplateSet {
     }
   }
 
-  mount(target: HTMLElement): Promise<InstanceType<T>> {
+  mount(target: HTMLElement, options?: MountOptions): Promise<InstanceType<T>> {
     if (!(target instanceof HTMLElement)) {
       throw new Error("Cannot mount component: the target is not a valid DOM element");
     }
@@ -56,7 +57,7 @@ export class App<T extends typeof Component = any> extends TemplateSet {
     }
     const node = new ComponentNode(this.Root, this.props, this);
     this.root = node;
-    return node.mountComponent(target);
+    return node.mountComponent(target, options);
   }
 
   destroy() {