feat(JitsiConference): add CONNECTION_ESTABLISHED event

Will emit CONNECTION_ESTABLISHED event to fill up the gap where the lib
users are not able to tell whether or not the connection has been
established if there were no interrupted/restored events emitted.

JitsiConference.js

             { value: establishmentDurationDiff });
     }
 
+    if (jingleSession.isP2P === this.isP2PActive()) {
+        this.eventEmitter.emit(JitsiConferenceEvents.CONNECTION_ESTABLISHED);
+    }
+
     if (done) {
 
         return;

JitsiConferenceEvents.js

  */
 export const CONFERENCE_LEFT = 'conference.left';
 
+/**
+ * Indicates that the connection to the conference has been established
+ * XXX This is currently fired whenVthe *ICE* connection enters 'connected'
+ * state for the first time.
+ */
+export const CONNECTION_ESTABLISHED = 'conference.connectionEstablished';
+
 /**
  * Indicates that the connection to the conference has been interrupted for some
  * reason.