Cleanup and overhaul (#21)

* Syntax cleanup

* More minor cleanup, use Linq

* Fix broken features by using correct values

* Feature flags the same

* Features are modular

* No AlphaFS, more .NET versions

* Fix appveyor

* Put back identifiers, for some reason

* String interpolation, modernization

* Better use of GetField

* XmlTextWriter to remove possible issues

* Fix header for OpenMSX

SabreTools.Library/DatFiles/Filter.cs

@@ -1,19 +1,16 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
+
 using SabreTools.Library.Data;
 using SabreTools.Library.DatItems;
 
-#if MONO
-using System.IO;
-#else
-using Alphaleonis.Win32.Filesystem;
-#endif
-
 namespace SabreTools.Library.DatFiles
 {
     /// <summary>
     /// Represents the filtering operations that need to be performed on a set of items, usually a DAT
     /// </summary>
+    /// TODO: Can this use `Field` instead of explicit filters?
     public class Filter
     {
         #region Pubically facing variables