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/NSApplication+Scriptability.swift

@@ -20,27 +20,74 @@ extension NSApplication : ScriptingObjectContainer {
         return "application"
     }
     
+    @objc(currentArticle)
+    func currentArticle() -> ScriptableArticle? {
+        var scriptableArticle: ScriptableArticle?
+        if let currentArticle = appDelegate.scriptingCurrentArticle {
+            if let feed = currentArticle.feed {
+                let scriptableFeed = ScriptableFeed(feed, container:self)
+                scriptableArticle = ScriptableArticle(currentArticle, container:scriptableFeed)
+            }
+        }
+        return scriptableArticle
+    }
+
+    @objc(selectedArticles)
+    func selectedArticles() -> NSArray {
+        let articles = appDelegate.scriptingSelectedArticles
+        let scriptableArticles:[ScriptableArticle] = articles.compactMap { article in
+            if let feed = article.feed  {
+                let scriptableFeed = ScriptableFeed(feed, container:self)
+                return ScriptableArticle(article, container:scriptableFeed)
+            } else {
+                return nil
+            }
+        }
+        return scriptableArticles as NSArray
+    }
+
+    // MARK: --- scriptable elements ---
+
     @objc(accounts)
     func accounts() -> NSArray {
         let accounts = AccountManager.shared.accounts
         return accounts.map { ScriptableAccount($0) } as NSArray
     }
     
+    @objc(valueInAccountsWithUniqueID:)
+    func valueInAccounts(withUniqueID id:String) -> ScriptableAccount? {
+        let accounts = AccountManager.shared.accounts
+        guard let account = accounts.first(where:{$0.accountID == id}) else { return nil }
+        return ScriptableAccount(account)
+    }
+
     /*
         accessing feeds from the application object skips the 'account' containment hierarchy
         this allows a script like 'articles of feed "The Shape of Everything"' as a shorthand
         for  'articles of feed "The Shape of Everything" of account "On My Mac"'
-    */
-    @objc(feeds)
-    func feeds() -> NSArray {
+    */  
+      
+    func allFeeds() -> [Feed] {
         let accounts = AccountManager.shared.accounts
         let emptyFeeds:[Feed] = []
-        let feeds = accounts.reduce(emptyFeeds) { (result, nthAccount) -> [Feed] in
+        return accounts.reduce(emptyFeeds) { (result, nthAccount) -> [Feed] in
               let accountFeeds = nthAccount.children.compactMap { $0 as? Feed }
               return result + accountFeeds
         }
+    }
+
+    @objc(feeds)
+    func feeds() -> NSArray {
+        let feeds = self.allFeeds()
         return feeds.map { ScriptableFeed($0, container:self) } as NSArray
     }
+
+    @objc(valueInFeedsWithUniqueID:)
+    func valueInFeeds(withUniqueID id:String) -> ScriptableFeed? {
+        let feeds = self.allFeeds()
+        guard let feed = feeds.first(where:{$0.feedID == id}) else { return nil }
+        return ScriptableFeed(feed, container:self)
+    }
 }