Started work on .slnx format support

[Wallby]

As per the CLA:

• minor parts of the code in this commit was copied from https://github.com/dotnet/roslyn, as suggested at Add .slnx solution format support #2703 by @JoeRobich
• all other code is entirely my own

What works:

• it compiles (the non-test binaries)
  What doesn't work:
• all automated tests

The actual functionality has not been tested yet. There are a few question marks that I have committed as-is:

• providing proper strings for project paths versus project names have not been worked out. I figured I'd first commit the structure to get feedback. I went over the Microsoft.VisualStudio.SolutionPersistence.Serializer implementation to figure out how to use it instead of the old text-parsing. The information for paths and names is available, so adjusting it I expect to not be a lot of work.
• exception handling has not been given much attention (e.g. things like Path.Combine), not sure if that is an issue for this project
• the ProjectIdInfo.SolutionConfigurations dictionary used to contain a different value. Currently it contains the same value as key. I was not entirely sure what to do about that. Are both parts even used anymore? Or can this be changed to an array of configuration strings? So feedback about that would be appreciated.

Given the force-pushing at #2703, I decided to start a separate pull request. Such that any changes to this pull request can be tracked.

[Wallby]

@microsoft-github-policy-service agree