Made a 2nd "_xna" .csproj file for the C# runtimes. This is unavoidable,...

... as microsoft is ass-backwards. The proper solution would be to add an "XNA" configuration (or even better, "XNA-XBOX" and "XNA-Windows" configuration), but Microsoft requires that any project referenced by XNA also import XNA, even if the base project doesn't depend on it.

Removed a lot of unneccessary stuff from the normal spine-csharp.csproj file, as no one else has Ultimate Edition and they were not configured properly.
Changed the XNA solution to point to the _xna csproj.