Confirm extension in use in sb2 serialization (#1643)

Add a check when serializing sb2 projects to see whether an extension is actually in use in a block or a visible monitor.

src/engine/runtime.js

@@ -1596,6 +1596,14 @@ class Runtime extends EventEmitter {
         return count;
     }
 
+    /**
+     *
+     * @return {OrderedMap} The current state of monitor blocks.
+     */
+    getMonitorState () {
+        return this._monitorState;
+    }
+
     /**
      * Queue monitor blocks to sequencer to be run.
      */