feat(external_api) add command and event listener for CS

modules/API/API.js

         case 'is-sharing-screen':
             callback(Boolean(APP.conference.isSharingScreen));
             break;
+        case 'get-content-sharing-participants': {
+            const tracks = getState()['features/base/tracks'];
+            const sharingParticipantIds = tracks.filter(tr => tr.videoType === 'desktop').map(t => t.participantId);
+
+            callback({
+                sharingParticipantIds
+            });
+            break;
+        }
         default:
             return false;
         }
         });
     }
 
+    /**
+     * Notify external application (if API is enabled) that the list of sharing participants changed.
+     *
+     * @param {Object} data - The event data.
+     * @returns {void}
+     */
+    notifySharingParticipantsChanged(data: Object) {
+        this._sendEvent({
+            name: 'content-sharing-participants-changed',
+            data
+        });
+    }
+
     /**
      * Notify external application (if API is enabled) that the device list has
      * changed.

modules/API/external/external_api.js

     'audio-availability-changed': 'audioAvailabilityChanged',
     'audio-mute-status-changed': 'audioMuteStatusChanged',
     'camera-error': 'cameraError',
+    'content-sharing-participants-changed': 'contentSharingParticipantsChanged',
     'device-list-changed': 'deviceListChanged',
     'display-name-change': 'displayNameChange',
     'email-change': 'emailChange',
         return getAvailableDevices(this._transport);
     }
 
+    /**
+     * Gets a list of the currently sharing participant id's.
+     *
+     * @returns {Promise} - Resolves with the list of participant id's currently sharing.
+     */
+    getContentSharingParticipants() {
+        return this._transport.sendRequest({
+            name: 'get-content-sharing-participants'
+        });
+    }
+
     /**
      * Returns Promise that resolves with current selected devices.
      *

react/features/base/tracks/middleware.js

     setTrackMuted
 } from './functions';
 
+import './subscriber';
+
 declare var APP: Object;
 
 /**

react/features/base/tracks/subscriber.js

+// @flow
+
+import _ from 'lodash';
+
+import { StateListenerRegistry } from '../../base/redux';
+
+declare var APP: Object;
+
+/**
+ * Notifies when the list of currently sharing participants changes.
+ */
+StateListenerRegistry.register(
+    /* selector */ state =>
+        state['features/base/tracks'].filter(tr => tr.videoType === 'desktop').map(t => t.participantId),
+    /* listener */ (participantIDs, store, previousParticipantIDs) => {
+        if (typeof APP !== 'object') {
+            return;
+        }
+
+        if (!_.isEqual(_.sortBy(participantIDs), _.sortBy(previousParticipantIDs))) {
+            APP.API.notifySharingParticipantsChanged(participantIDs);
+        }
+    }
+);