feat(external_api) add event for chat updates (unread counter, open state)

modules/API/API.js

         }
     }
 
+    /**
+     * Notify external application (if API is enabled) that the chat state has been updated.
+     *
+     * @param {number} unreadCount - The unread messages counter.
+     * @param {boolean} isOpen - True if the chat panel is open.
+     * @returns {void}
+     */
+    notifyChatUpdated(unreadCount: number, isOpen: boolean) {
+        this._sendEvent({
+            name: 'chat-updated',
+            unreadCount,
+            isOpen
+        });
+    }
+
     /**
      * Notify external application (if API is enabled) that message was sent.
      *

modules/API/external/external_api.js

     'audio-availability-changed': 'audioAvailabilityChanged',
     'audio-mute-status-changed': 'audioMuteStatusChanged',
     'camera-error': 'cameraError',
+    'chat-updated': 'chatUpdated',
     'content-sharing-participants-changed': 'contentSharingParticipantsChanged',
     'device-list-changed': 'deviceListChanged',
     'display-name-change': 'displayNameChange',
      * logLevel: the message log level
      * arguments: an array of strings that compose the actual log message
      * }}
+     * {@code chatUpdated} - receives event notifications about chat state being
+     * updated. The listener will receive object with the following structure:
+     * {{
+     *  'unreadCount': unreadCounter, // the unread message(s) counter,
+     *  'isOpen': isOpen, // whether the chat panel is open or not
+     * }}
      * {@code incomingMessage} - receives event notifications about incoming
      * messages. The listener will receive object with the following structure:
      * {{

react/features/chat/middleware.js

 import { playSound, registerSound, unregisterSound } from '../base/sounds';
 import { showToolbox } from '../toolbox/actions';
 
-import { SEND_MESSAGE, SET_PRIVATE_MESSAGE_RECIPIENT } from './actionTypes';
+import { ADD_MESSAGE, TOGGLE_CHAT, SEND_MESSAGE, SET_PRIVATE_MESSAGE_RECIPIENT } from './actionTypes';
 import { addMessage, clearMessages, toggleChat } from './actions';
 import { ChatPrivacyDialog } from './components';
 import {
     MESSAGE_TYPE_LOCAL,
     MESSAGE_TYPE_REMOTE
 } from './constants';
+import { getUnreadCount } from './functions';
 import { INCOMING_MSG_SOUND_FILE } from './sounds';
 
 declare var APP: Object;
  * @returns {Function}
  */
 MiddlewareRegistry.register(store => next => action => {
-    const { dispatch } = store;
+    const { dispatch, getState } = store;
+    let isOpen, unreadCount;
 
     switch (action.type) {
+    case ADD_MESSAGE:
+        unreadCount = action.hasRead ? 0 : getUnreadCount(getState()) + 1;
+        isOpen = getState()['features/chat'].isOpen;
+
+        if (typeof APP !== 'undefined') {
+            APP.API.notifyChatUpdated(unreadCount, isOpen);
+        }
+        break;
+    case TOGGLE_CHAT:
+        unreadCount = 0;
+        isOpen = !getState()['features/chat'].isOpen;
+
+        if (typeof APP !== 'undefined') {
+            APP.API.notifyChatUpdated(unreadCount, isOpen);
+        }
+        break;
     case APP_WILL_MOUNT:
         dispatch(
                 registerSound(INCOMING_MSG_SOUND_ID, INCOMING_MSG_SOUND_FILE));