Write a better Per-Monitor DPI Sample

.NET 4.8 made a bunch of improvements wrt per-monitor DPI, and can now support _mixed-mode_ DPI (system-aware child-windows in per-monitor aware top-level windows). These improvements are also inherited by .NET Core. 

Improve the guide at https://github.com/Microsoft/WPF-Samples/blob/master/PerMonitorDPI/readme.md, and also write a better sample to go with it. 