PreviewThumbnails constructor.
@param {Plyr} player
@return {PreviewThumbnails}
function PreviewThumbnails(player) {
	    _classCallCheck(this, PreviewThumbnails);

	    this.player = player;
	    this.thumbnails = [];
	    this.loaded = false;
	    this.lastMouseMoveTime = Date.now();
	    this.mouseDown = false;
	    this.loadedImages = [];
	    this.elements = {
	      thumb: {},
	      scrubbing: {}
	    };
	    this.load();
	  }