adding ability for IFileSystem to tell me when something changes on disk.

first pass at implementing the windows version of the new interface methods
IViewPort now has a public method to call whenever you want to notify it that its underlying file has been changed.
Added 3 tests showing how the Editor and ViewPort should behave with this new functionality.
- Editor should hook up ViewPorts whenever a new tab is opened
- Editor should clear out ViewPorts whenever a tab is closed
- ViewPort should reload when the file is changed and it is not.

Note that the model isn't implemented yet. This is just some failing tests.

Gen3Hex/Model/IFileSystem.cs

@@ -1,4 +1,6 @@
-namespace HavenSoft.Gen3Hex.Model {
+using System;
+
+namespace HavenSoft.Gen3Hex.Model {
    public interface IFileSystem {
       string CopyText { get; set; }
 
@@ -34,6 +36,22 @@
       /// </returns>
       LoadedFile OpenFile(params string[] extensionOptions);
 
+      /// <summary>
+      /// Have the filesystem open a specific file.
+      /// </summary>
+      /// <returns>
+      /// If the file exists, it is loaded and returned.
+      /// If it doesn't exist, returns null.
+      /// </returns>
+      LoadedFile LoadFile(string fileName);
+
+      /// <summary>
+      /// When a file changes, the filesystem will call all listeners for that file.
+      /// </summary>
+      void AddListenerToFile(string fileName, Action<IFileSystem> listener);
+
+      void RemoveAllListenersForFile(string fileName);
+
       /// <summary>
       /// Saves the file without prompting the user for permission.
       /// </summary>