Extend MSBuild binding mechanism to solution properties

[glopesdev]

In the article https://github.com/dotnet/templating/wiki/Binding-and-project-context-evaluation#binding-to-msbuild-properties it is mentioned that "It is possible to bind symbols to MSBuild properties of the current project."

Upon testing I realized that this really only works for templates running inside a project folder, but will not work for templates running inside the root solution folder.

This would be useful to access properties in Directory.Build.props for example for creating documentation pages or other top-level content items which might benefit from accessing global solution properties such as <Authors> or <Copyright>.