ref(LargeVideo): rename to LargeVideoManager

It is confusing when the name of the main class exported from the file
is not the same as the filename.

modules/UI/videolayout/VideoLayout.js

 import UIUtil from "../util/UIUtil";
 
 import RemoteVideo from "./RemoteVideo";
-import LargeVideoManager  from "./LargeVideo";
+import LargeVideoManager  from "./LargeVideoManager";
 import {VIDEO_CONTAINER_TYPE} from "./VideoContainer";
 import {SHARED_VIDEO_CONTAINER_TYPE} from '../shared_video/SharedVideo';
 import LocalVideo from "./LocalVideo";