Fix _removeThread documentation and add toggleStack

src/engine/thread.js

@@ -4,6 +4,10 @@
  * @constructor
  */
 function Thread (firstBlock) {
+    /**
+     * Top block of the thread
+     */
+    this.topBlock = firstBlock;
     /**
      * Next block that the thread will execute.
      * @type {string}