Add support for .NET 6

[brantburnett]

Motivation

Allow Windows consumers to use .NET 6 or later instead of .NET 4.

Modifications

• Move to the modern .csproj format with multi-targeting
• Drop the .nuspec file and instead add properties directly to the csproj
• Update to latest C++ platform toolset
• Remove the unused settings files
• Fix some miscellaneous warnings and compatibility issues
• Set Visible to False for the DLLs in the targets file so they don't appear in Visual Studio and only get copied to output
• Use buildTransitive so that when WPFMediaKit is a transitive dependency the required DLLs are still copied
• Add SourceLink support and produce snupkg symbol packages
• Embed the license and readme in the package
• Bump version to 2.3.0

Results

The NuGet package can now be produced by the .NET SDK 6 or greater with:

dotnet pack -c Release .\Source

The resulting package includes .NET 4.6.2 targets (oldest framework still in support by Microsoft) and .NET 6 targets.

[ckeisc]

Thanks for doing this. I made use of this in my hobby project! The upstream repo has not updated for years, so I wonder if this would be merged. If not, do consider a fork! Thanks again.