rename 'UpdateThread' structure to 'Updater', because it is no longer a separate thread

shell/updater.sml

@@ -0,0 +1,30 @@
+structure Updater =
+struct
+  open MailboxType
+  open InputMsg
+
+  fun sendMsg msg =
+    case msg of
+      DRAW msg => DrawMailbox.append msg
+    | SEARCH (buffer, searchString, time) =>
+        Mailbox.send (SearchMailbox.mailbox, (buffer, searchString, time))
+
+  fun sendMsgs msgList =
+    case msgList of
+      hd :: tl => let val () = sendMsg hd in sendMsgs tl end
+    | [] => ()
+
+  fun update (app: AppType.app_type, inputMsg) =
+    let
+      val time = Time.now ()
+
+      val () = ExceptionLogger.addCommand inputMsg
+
+      val app = AppUpdate.update (app, inputMsg, time)
+                handle e => ExceptionLogger.log e
+
+      val () = sendMsgs (#msgs app)
+    in
+      app
+    end
+end