Add PackageLicenseExpression and cleanup project properties

[lahma]

License expression is the preferred way (shows in NuGet.org and used in license validations). I created root level Directory.Build.props that projects will inherit so no need to have same info (in different ways) in all projects. For next release you just need to update the version number in root Directory.Build.props.

dotnet build -c Release && dotnet pack -c Release

Now produces NuGet packages like this: