new folder structure and new C# script host

src/Misc/Extension.cs

@@ -0,0 +1,65 @@
+
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.Composition;
+using System.ComponentModel.Composition.Hosting;
+using System.IO;
+using System.Linq;
+
+using static mpvnet.Core;
+
+namespace mpvnet
+{
+    public class Extension
+    {
+        [ImportMany]
+        public IEnumerable<IExtension> Extensions = null;
+
+        readonly CompositionContainer CompositionContainer;
+
+        public Extension()
+        {
+            try
+            {
+                AggregateCatalog catalog = new AggregateCatalog();
+                string dir = Folder.Startup + "Extensions";
+
+                if (Directory.Exists(dir))
+                {
+                    string[] knownExtensions = { "RatingExtension", "ScriptingExtension" };
+
+                    foreach (string extDir in Directory.GetDirectories(dir))
+                    {
+                        if (knownExtensions.Contains(Path.GetFileName(extDir)))
+                            catalog.Catalogs.Add(new DirectoryCatalog(extDir, Path.GetFileName(extDir) + ".dll"));
+                        else
+                            ConsoleHelp.WriteError("Failed to load extension:\n\n" +  extDir +
+                                "\n\nOnly extensions that ship with mpv.net are allowed in <startup>\\extensions" +
+                                "\n\nUser extensions have to use <config folder>\\extensions" +
+                                "\n\nNever copy or install a new mpv.net version over a old mpv.net version.");
+                    }
+                }
+
+                dir = core.ConfigFolder + "extensions";
+
+                if (Directory.Exists(dir))
+                    foreach (string extDir in Directory.GetDirectories(dir))
+                        catalog.Catalogs.Add(new DirectoryCatalog(extDir, Path.GetFileName(extDir) + ".dll"));
+
+                if (catalog.Catalogs.Count > 0)
+                {
+                    CompositionContainer = new CompositionContainer(catalog);
+                    CompositionContainer.ComposeParts(this);
+                }
+            }
+            catch (Exception ex)
+            {
+                App.ShowException(ex);
+            }
+        }
+    }
+
+    public interface IExtension
+    {
+    }
+}