* FIXED: Incorrect project instances for the active solution configuration in XProject Environment.
Related issue: https://github.com/3F/vsSolutionBuildEvent/issues/71
* FIXED: Fixed possible 'The path is not of a legal form.' when instantiating ProjectItem.
* NEW: Added .IsVc() extension method for ProjectItem objects.
* CHANGED: Updated logic for .IsSdk() extension method.
Now it will use information from ProjectType when null or empty ProjectItem.fullPath.
Otherwise, same logic from v2.5.3 described here:
https://github.com/3F/MvsSln/blob/0ec96021b7/MvsSln/Extensions/ProjectItemExtension.cs#L59-L77
