Implement external API notification about screen sharing status

conference.js

             replaceLocalTrack(this.localVideo, newStream, room))
             .then(() => {
                 this.localVideo = newStream;
-
+                this._setSharingScreen(newStream);
                 if (newStream) {
-                    this.isSharingScreen = newStream.videoType === 'desktop';
-
                     APP.UI.addLocalStream(newStream);
-                } else {
-                    this.isSharingScreen = false;
                 }
                 this.setVideoMuteStatus(this.isLocalVideoMuted());
-                APP.UI.updateDesktopSharingButtons();
             });
     },
 
+    /**
+     * Sets `this.isSharingScreen` depending on provided video stream.
+     * In case new screen sharing status is not equal previous one
+     * it updates desktop sharing buttons in UI
+     * and notifies external application.
+     *
+     * @param {JitsiLocalTrack} [newStream] new stream to use or null
+     * @private
+     * @returns {void}
+     */
+    _setSharingScreen(newStream) {
+        const wasSharingScreen = this.isSharingScreen;
+
+        this.isSharingScreen = newStream && newStream.videoType === 'desktop';
+
+        if (wasSharingScreen !== this.isSharingScreen) {
+            APP.UI.updateDesktopSharingButtons();
+            APP.API.notifyScreenSharingStatusChanged(this.isSharingScreen);
+        }
+    },
+
     /**
      * Start using provided audio stream.
      * Stops previous audio stream.

doc/api.md

 }
 ```
 
+* **screenSharingStatusChanged** - receives event notifications about turning on/off the local user screen sharing. The listener will receive object with the following structure:
+```javascript
+{
+"on": on //whether screen sharing is on
+}
+```
+
 * **incomingMessage** - Event notifications about incoming
 messages. The listener will receive an object with the following structure:
 ```javascript

modules/API/API.js

         this._sendEvent({ name: 'feedback-submitted' });
     }
 
+    /**
+     * Notify external application (if API is enabled) that the screen sharing
+     * has been turned on/off.
+     *
+     * @param {boolean} on - True if screen sharing is enabled.
+     * @returns {void}
+     */
+    notifyScreenSharingStatusChanged(on: boolean) {
+        this._sendEvent({
+            name: 'screen-sharing-status-changed',
+            on
+        });
+    }
+
     /**
      * Disposes the allocated resources.
      *

modules/API/external/external_api.js

     'video-conference-joined': 'videoConferenceJoined',
     'video-conference-left': 'videoConferenceLeft',
     'video-availability-changed': 'videoAvailabilityChanged',
-    'video-mute-status-changed': 'videoMuteStatusChanged'
+    'video-mute-status-changed': 'videoMuteStatusChanged',
+    'screen-sharing-status-changed': 'screenSharingStatusChanged'
 };
 
 /**
      * {{
      * roomName: room //the room name of the conference
      * }}
+     * screenSharingStatusChanged - receives event notifications about
+     * turning on/off the local user screen sharing.
+     * The listener will receive object with the following structure:
+     * {{
+     * on: on //whether screen sharing is on
+     * }}
      * readyToClose - all hangup operations are completed and Jitsi Meet is
      * ready to be disposed.
      * @returns {void}