Community Toolkit Support

[alexhelms]

@canton7 and I had a chat on the C# discord about Community Toolkit support in Stylet.

To summarize:

• Change Screen to derive from ObservableValidator which would remove Stylet implementations of INPC and INotifyDataErrorInfo.
• Stylet's built-in validation would now be replaced with the validation scheme used by Community Toolkit.
• PropertyChangedDispatcher concept would be removed.
• Fix up the tests.

Also part of this (not discussed on discord) would be to bump the minimum dotnet versions of stylet to .NET Framework 4.6.2 and dotnet 6/8. This is required to use Community Toolkit.

I've made the changes and have opened #397.