feat(external_api): notify when api is disposed

doc/api.md

 
 * **readyToClose** - event notification fired when Jitsi Meet is ready to be closed (hangup operations are completed).
 
-* **subjectChange** - event notifications about subject of conference changes. 
+* **subjectChange** - event notifications about subject of conference changes.
 The listener will receive an object with the following structure:
 ```javascript
 {

modules/API/external/external_api.js

      * @returns {void}
      */
     dispose() {
+        this.emit('_willDispose');
         this._transport.dispose();
         this.removeAllListeners();
         if (this._frame) {