fix(caps): add event to notify when caps version for a user is changed

JitsiConferenceEventManager.js

  */
 JitsiConferenceEventManager.prototype.setupXMPPListeners = function () {
     var conference = this.conference;
+    conference.xmpp.caps.addListener(XMPPEvents.PARTCIPANT_FEATURES_CHANGED,
+        from => {
+            const participant = conference.getParticipantId(
+                Strophe.getResourceFromJid(from));
+            if(participant) {
+                conference.eventEmitter.emit(
+                    JitsiConferenceEvents.PARTCIPANT_FEATURES_CHANGED,
+                    participant);
+            }
+        });
     conference.xmpp.addListener(
         XMPPEvents.CALL_INCOMING, conference.onIncomingCall.bind(conference));
     conference.xmpp.addListener(

JitsiConferenceEvents.js

  */
 export const PARTICIPANT_CONN_STATUS_CHANGED
     = "conference.participant_conn_status_changed";
+/**
+ * Indicates that the features of the participant has been changed.
+ */
+export const PARTCIPANT_FEATURES_CHANGED
+    = "conference.partcipant_features_changed";
 /**
  * Indicates that a the value of a specific property of a specific participant
  * has changed.

modules/xmpp/Caps.js

 /* global $, b64_sha1, Strophe */
 import XMPPEvents from "../../service/xmpp/XMPPEvents";
+import Listenable from "../util/Listenable";
 
 /**
  * The property
 /**
  * Implements xep-0115 ( http://xmpp.org/extensions/xep-0115.html )
  */
-export default class Caps {
+export default class Caps extends Listenable {
     /**
      * Constructs new Caps instance.
      * @param {Strophe.Connection} connection the strophe connection object
      * that will be sent to the other participants
      */
     constructor(connection = {}, node = "http://jitsi.org/jitsimeet") {
+        super();
         this.node = node;
         this.disco = connection.disco;
         if(!this.disco) {
         const caps = stanza.querySelector("c");
         const version = caps.getAttribute("ver");
         const node = caps.getAttribute("node");
+        const oldVersion = this.jidToVersion[from];
         this.jidToVersion[from] = {version, node};
+        if(oldVersion && oldVersion.version !== version) {
+            this.eventEmitter.emit(XMPPEvents.PARTCIPANT_FEATURES_CHANGED,
+                from);
+        }
         // return true to not remove the handler from Strophe
         return true;
     }

service/xmpp/XMPPEvents.js

     // Note: currently this event fires every time we receive presence from
     // someone (regardless of whether or not the "video type" changed).
     PARTICIPANT_VIDEO_TYPE_CHANGED: "xmpp.video_type",
+    /**
+     * Indicates that the features of the participant has been changed.
+     */
+    PARTCIPANT_FEATURES_CHANGED: "xmpp.partcipant_features_changed",
     PASSWORD_REQUIRED: "xmpp.password_required",
     PEERCONNECTION_READY: "xmpp.peerconnection_ready",
     /**