implement 'current article' property of the application

Also, fix the unique-id based accessors (the four byte code  for id was
wrong in the sdef)
Add valueIn<Key>WithUniqueID accessors
Add a few protocols and protocol implementations for the AppDelegate
and MainWindowControllor so as to expose needed functionality for
scriptability

Evergreen/Scriptability/AppDelegate+Scriptability.swift

@@ -0,0 +1,28 @@
+//
+//  AppDelegate+Scriptability.swift
+//  Evergreen
+//
+//  Created by Olof Hellman on 2/7/18.
+//  Copyright © 2018 Ranchero Software. All rights reserved.
+//
+
+/*
+    Note:  strictly, the AppDelegate doesn't appear as part of the scripting model,
+    so this file is rather unlike the other Object+Scriptability.swift files.
+    However, the AppDelegate object is the de facto scripting accessor for some
+    application elements and properties.  For, example, the main window is accessed
+    via the AppDelegate's MainWindowController, and the main window itself has
+    selected feeds, selected articles and a current article.  This file supplies the glue to access
+    these scriptable objects, while being completely separate from the core AppDelegate code,
+*/
+
+import Foundation
+import Data
+
+protocol ScriptingAppDelegate {
+    var  scriptingCurrentArticle: Article?  {get}
+    var  scriptingSelectedArticles: [Article]  {get}
+    var  scriptingMainWindowController:ScriptingMainWindowController? {get}
+}
+
+