Add label and dash for no data

I noticed that there was an unused UILabel, so it's being updated and I added a property to display it.

I also found it useful when there wasn't any data (or invalid data) to show just a dash [–], so that option is also included.

There's a few tweaks to the example so that thes changes can be seen: A label is enabled on one progress indicator and a slight pause at 100.