System.Waf

• WAF: WPF libraries added support for .Net Core 3.1.
• WAF: Add nullable annotations which are used by C# 8.0 Nullable Reference Types support.
• WAF Remove obsolete method CollectionHelper.Empty (Breaking Change).
• WAF: WPF unit tests run now on .NET Core 3.1 and on .NET Framework 4.6.1.
• BookLibrary: Migrate Entity Framework Core 2.2 to Entity Framework Core 3.1.
• Writer: Move creating the XAML print preview into Presentation layer as it depends on the Dispatcher thread.
• Samples: Target multiple frameworks (runtimes): .Net Core 3.1 and .NET Framework 4.7.2.
• Samples: Run unit tests on all targeted frameworks: .Net Core 3.1 and .NET Framework 4.7.2.
• All: Migrate all projects (csproj) to the new Project SDK format.
• All: Improve code by using C# 8.0.
• Build: Visual Studio 2019 (Version 16.4) or newer is required.