[FIX] app: make subroots more robust

This commit fixes two issues with subroots:

1. creating a subroot create a new component node synchronously. This
   would causes issues if the creation was done in the setup of a
component, since in that case, owl would reset the current component to
null right after, which would cause all calls to hooks to fail. This is
fixed by restoring the previous component node right after creating a
root.

2. the destroy method for roots calls the scheduler processTasks.
   However, the processTasks method was not safe to reentrant calls,
which would in some cases crashes owl. For example, if a destroy is done
while a new component is mounted, the mount method would be called
twice.

This is fixed by ignoring the processTasks if we are currently
processing tasks. It works because the "for ... of" loop will still
process all new tasks in the current iteration.

tests/app/app.test.ts

@@ -1,4 +1,4 @@
-import { App, Component, mount, onWillStart, useState, xml } from "../../src";
+import { App, Component, mount, onWillPatch, onWillStart, useState, xml } from "../../src";
 import { status } from "../../src/runtime/status";
 import {
   makeTestFixture,
@@ -184,4 +184,21 @@ describe("app", () => {
     expect(Object.keys(app.templates)).toEqual(["hello"]);
     expect(Object.keys(app.rawTemplates)).toEqual(["hello", "world"]);
   });
+
+  test("can call processTask twice in a row without crashing", async () => {
+    class Child extends Component {
+      static template = xml`<div/>`;
+      setup() {
+        onWillPatch(() => app.scheduler.processTasks());
+      }
+    }
+    class SomeComponent extends Component {
+      static template = xml`parent<Child/>`;
+      static components = { Child };
+    }
+
+    const app = new App(SomeComponent);
+    await app.mount(fixture);
+    expect(fixture.innerHTML).toBe("parent<div></div>");
+  });
 });