[FIX] props validation: does not crash with t-call-context

The code for props validation assumed that the rendering context was a
component.  This was actually true when it was written, but is no longer
true since t-call-context was introduced.

Because of that, it would crash when trying to access the internals of
the component, such as the static components object.

The fix is simple: instead of passing the context to the props
validation code, which can now be anything, we pass the component node,
which is guaranteed to give a reference to the component (and also to
the app).  This also make the code slightly simpler.

closes #1261

src/runtime/app.ts

@@ -66,7 +66,7 @@ export class App<
   mount(target: HTMLElement, options?: MountOptions): Promise<Component<P, E> & InstanceType<T>> {
     App.validateTarget(target);
     if (this.dev) {
-      validateProps(this.Root, this.props, { __owl__: { app: this } });
+      validateProps(this.Root, this.props, { app: this });
     }
     const node = this.makeNode(this.Root, this.props);
     const prom = this.mountNode(node, target, options);