Get participant specific video element

* Get participant specific video element

We now have the ability to select the video element for specific participants. I'm tweaking the jitsi-meet-electron app for my use case. I need to open Always On Top windows for specific participants, so the current _getLargeVideo() wont suffice.

I made a post about this in the Developers section on the Jitsi Community Forum, but it got blocked by Akismet.

* Add dots at end of sentence.

* Fixed ESlint errors and add additional check for iframe.

* Use _myUserID instead of string.

* Return the local video by default if participantId is undefined.

* Fixed mistake in string template.

modules/API/external/external_api.js

         return iframe.contentWindow.document.getElementById('largeVideo');
     }
 
+    /**
+     * Getter for participant specific video element in Jitsi Meet.
+     *
+     * @param {string|undefined} participantId - Id of participant to return the video for.
+     *
+     * @returns {HTMLElement|undefined} - The requested video. Will return the local video
+     * by default if participantId is undefined.
+     */
+    _getParticipantVideo(participantId) {
+        const iframe = this.getIFrame();
+
+        if (!iframe
+                || !iframe.contentWindow
+                || !iframe.contentWindow.document) {
+            return;
+        }
+
+        if (typeof participantId === 'undefined' || participantId === this._myUserID) {
+            return iframe.contentWindow.document.getElementById('localVideo_container');
+        }
+
+        return iframe.contentWindow.document.querySelector(`#participant_${participantId} video`);
+    }
+
     /**
      * Sets the size of the iframe element.
      *