SlnFile should preserve the order of SlnSections

[KirillOsenkov]

When writing out the .sln it should attempt to preserve the order of SlnSections as it found them in the original file.

[abock]

It should mostly preserve section ordering. There is a bit of an issue in the way SlnFile is implemented however which does force ordering. I can't really do anything about that without changing some core bits of SlnFile and my desire is to keep it as close to MD/.NET Core as possible. I can provide some more detail next week.