findTrack(timeline, trackId) {
		let track = timeline.video.find(track => (track.id === trackId));
		if (track === undefined) {
			track = timeline.audio.find(track => (track.id === trackId));
		}
		return track;
	}
Get track with specified trackId

@param {Object} timeline Object with 'video' and 'audio' entries.
@param {string} trackId
@return {Object|undefined}