Adding isSharingScreen() to external API

modules/API/API.js

         case 'is-video-available':
             callback(videoAvailable);
             break;
+        case 'is-sharing-screen':
+            callback(Boolean(APP.conference.isSharingScreen));
+            break;
         default:
             return false;
         }

modules/API/external/external_api.js

         });
     }
 
+    /**
+     * Returns screen sharing status.
+     *
+     * @returns {Promise} - Resolves with screensharing status and rejects on failure.
+     */
+    isSharingScreen() {
+        return this._transport.sendRequest({
+            name: 'is-sharing-screen'
+        });
+    }
+
     /**
      * Returns the avatar URL of a participant.
      *