Solution file parser should parse and preserve ProjectSection(SolutionItems)

Currently the solution parser just skips over solution items, here's the code:
http://source.dot.net/#Microsoft.Build/Construction/Solution/SolutionFile.cs,775

Since the solution parser is the only public and official .sln parser it'd be nice if it supported parsing solution items, which is easy to add. Here's an example syntax:

```
Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{E3D2D556-C8D6-4107-902F-5092299605DC}"
ProjectSection(SolutionItems) = preProject
nuget.config = nuget.config
tools.proj = tools.proj
EndProjectSection
EndProject

```