Refactored WebRTC screenshare flow and fixed mcs-core event emission

bigbluebutton-client/resources/prod/lib/kurento-extension.js

...luebutton-html5/client/compatibility/kurento-extension.js

labs/bbb-webrtc-sfu/lib/base/BaseManager.js

labs/bbb-webrtc-sfu/lib/bbb/messages/Constants.js

labs/bbb-webrtc-sfu/lib/mcs-core/lib/media/MCSApiStub.js

.../bbb-webrtc-sfu/lib/mcs-core/lib/media/MediaController.js

labs/bbb-webrtc-sfu/lib/mcs-core/lib/model/MediaSession.js

labs/bbb-webrtc-sfu/lib/mcs-core/lib/model/SfuUser.js

labs/bbb-webrtc-sfu/lib/screenshare/ScreenshareManager.js

labs/bbb-webrtc-sfu/lib/screenshare/screenshare.js

labs/bbb-webrtc-sfu/lib/video/VideoManager.js