RootLayout overlays: open/close in the main window

Make the app root a RootLayout (it extends GridLayout) and any code can float views over the current page — no modal window, no navigation:

<!-- app-root.xml -->
<RootLayout>
  <Frame defaultPage="main-page" />
</RootLayout>

import { Color, CoreTypes, getRootLayout, Label } from '@nativescript/core';

export async function showToast(message: string) {
  const toast = new Label();
  toast.text = message;
  toast.verticalAlignment = CoreTypes.VerticalAlignment.bottom;
  toast.marginBottom = 48;