Add dotnet 6.0 support.

[peymanr34]

Closes: #222, Closes: #226, Closes: #229

Summary:

• ImageListView and WPFThumbnailExtractor projects has been migrated to the sdk-style project format.
• Target frameworks has been set to net35 and net6.0 to stay compatible with older versions of dotnet.
• The DocumentationFile properties has been replaced with GenerateDocumentationFile property.
• The CodeAnalysisRuleSet properties has been replaced with .NET Analyzers.

Current issues

• Packing the library using dotnet pack command fails.

dotnet pack .\ImageListView\ImageListView.csproj -c Release

error : ResGen.exe not supported on .NET Core MSBuild

• Packing the sdk-style projects using nuget pack command is not supported.

I think package properties should be moved from ImageListView.nuspec to ImageListView.csproj (or alternatively Directory.Build.props) then pack it via dotnet pack or msbuild -t:pack.

That being said, I'm not exactly sure how to include WPFThumbnailExtractor.dll into the package.

Out of scope

Serving the documentations fails with the following error which can be solved by updating the docfx.console nuget package.

The SDK 'Microsoft.NET.Sdk' specified could not be found.