Fix Unity project generation with .NET Standard

ProjectFileHook.cs

using System.IO;
using System.Linq;
using System.Text;
using System.Xml.Linq;

using UnityEditor;

#if ENABLE_VSTU

using SyntaxTree.VisualStudio.Unity.Bridge;

[InitializeOnLoad]
public class ProjectFileHook
{
	// necessary for XLinq to save the xml project file in utf8
	class Utf8StringWriter : StringWriter
	{
		public override Encoding Encoding
		{
			get { return Encoding.UTF8; }
		}
	}

	static ProjectFileHook()
	{
		ProjectFilesGenerator.ProjectFileGeneration += (string name, string content) =>
		{
			// parse the document and make some changes
			var document = XDocument.Parse(content);
			var ns = document.Root.Name.Namespace;

			document.Root
				.Descendants()
				.First(x => x.Name.LocalName == "PropertyGroup")
				.Add(new XElement(ns + "ImplicitlyExpandNETStandardFacades", "false"), 
				     new XElement(ns + "ImplicitlyExpandDesignTimeFacades", "false"));

			// save the changes using the Utf8StringWriter
			var str = new Utf8StringWriter();
			document.Save(str);

			return str.ToString();
		};
	}
}

#endif