Align MSBuild path casing with the repository

MSBuild resolves paths case-insensitively on Windows, so a solution can reference assemblies/WPFToolkit/WPFToolkit.csproj while the directory committed to git is actually named assemblies/Wpftoolkit. The same reference fails with MSB3202 or Project file not found on a case-sensitive file system. This recipe rewrites path references in .sln and .slnx solutions, in .props and .targets files, and in MSBuild project files such as .csproj, .vbproj, .fsproj, .vcxproj, .sqlproj, .shproj, and .projitems, so that every segment matches the casing of the file or directory that is actually in the repository. References that already resolve exactly, that cannot be resolved at all, or whose casing is ambiguous — because two files or directories differ only by case — are left untouched.