Hide beat markers

Going along with the theme of hiding things that make things too easy (ie the BPM display), it would also be nice to be able to hide the beat markers (the red and grey lines).

It's too easy to just look at them and see where the issue is, instead of using your ears to train yourself better.

I can do the whole "stick a label over the BPM readout" (which isn't ideal) - but no way to hide the markers!