Adds uiLoaded event in iframe API, fired when all resources are loaded.

doc/api.md

     * **interfaceConfigOverwrite**: (optional) JS object with overrides for options defined in [interface_config.js].
     * **noSSL**: (optional, defaults to true) Boolean indicating if the server should be contacted using HTTP or HTTPS.
     * **jwt**: (optional) [JWT](https://jwt.io/) token.
+    * **onload**: (optional) handler for the iframe onload event.
 
 Example:
 

modules/API/external/external_api.js

             configOverwrite,
             interfaceConfigOverwrite,
             noSSL,
-            jwt
+            jwt,
+            onload
         ] = args;
 
         return {
             configOverwrite,
             interfaceConfigOverwrite,
             noSSL,
-            jwt
+            jwt,
+            onload
         };
     case 'object': // new arguments format
         return args[0];
      * used.
      * @param {string} [options.jwt] - The JWT token if needed by jitsi-meet for
      * authentication.
+     * @param {string} [options.onload] - The onload function that will listen
+     * for iframe onload event.
      */
     constructor(domain, ...args) {
         super();
             configOverwrite = {},
             interfaceConfigOverwrite = {},
             noSSL = false,
-            jwt = undefined
+            jwt = undefined,
+            onload = undefined
         } = parseArguments(args);
 
         this._parentNode = parentNode;
             noSSL,
             roomName
         });
-        this._createIFrame(height, width);
+        this._createIFrame(height, width, onload);
         this._transport = new Transport({
             backend: new PostMessageTransportBackend({
                 postisOptions: {
      * parseSizeParam for format details.
      * @param {number|string} width - The with of the iframe. Check
      * parseSizeParam for format details.
+     * @param {Function} onload - The function that will listen
+     * for onload event.
      * @returns {void}
      *
      * @private
      */
-    _createIFrame(height, width) {
+    _createIFrame(height, width, onload) {
         const frameName = `jitsiConferenceFrame${id}`;
 
         this._frame = document.createElement('iframe');
         this._setSize(height, width);
         this._frame.setAttribute('allowFullScreen', 'true');
         this._frame.style.border = 0;
+
+        if (onload) {
+            // waits for iframe resources to load
+            // and fires event when it is done
+            this._frame.onload = onload;
+        }
+
         this._frame = this._parentNode.appendChild(this._frame);
     }