Issue1045 - Wrapped duration widgets' Text widgets in a stack and added transparent Texts representing largest possible time string - DC

[ChopinDavid]

This would close #1045

It is worth noting that this will have some merge conflicts with #1044, should that PR be merged.
If #1044 does end up being merged, we would want to _CurrentPositionState's and _RemainingDurationState's build methods to look like

@override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        Text(
          "${_controller.value.position.inHours != 0 ? '00:' : ''}00:00",
          key: const Key('hidden-current-position-text'),
          style: const TextStyle(
            color: Colors.transparent,
            fontSize: 12.0,
            height: 1.0,
          ),
        ),
        Text(
          durationFormatter(
            _controller.value.position.inMilliseconds,
          ),
          style: const TextStyle(
            color: Colors.white,
            fontSize: 12.0,
            height: 1.0,
          ),
        ),
      ],
    );
  }

and

@override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        Text(
          "- ${_controller.value.position.inHours != 0 ? '00:' : ''}00:00",
          key: const Key('hidden-remaining-duration-text'),
          style: const TextStyle(
            color: Colors.transparent,
            fontSize: 12.0,
            height: 1.0,
          ),
        ),
        Text(
          "- ${durationFormatter(
            (_controller.metadata.duration.inMilliseconds) -
                (_controller.value.position.inMilliseconds),
          )}",
          style: const TextStyle(
            color: Colors.white,
            fontSize: 12.0,
            height: 1.0,
          ),
        ),
      ],
    );
  }

, respectively.