Fix debug assertion caused by BpmControl

[uklotzde]

Occurs when tapping the bpm with "Assume constant tempo" disabled.

[Holzhaus]

Code looks good, but didn't test.

I'm unsure if ignoring the tapping is correct, because it's a bit unexpected from the user's perspective. We could also throw away the beatmap and replace it with a beatgrid that has the tapped Bpm. But that's debatable and I don't want to add even more changes here, when #2961 gets rid of the beatscapabilities stuff anyway.

[daschuer]

The Tap feature was disabled in the beats track preference for beats maps. The user has to cklick assume const first to enable the button.

But since it is a CO we cannot disable the button on the controllers.

Here we have the trade of protecting the user from his faults versus doing what the user expects.

The debug assertion has made a behavior visible that was always there. We had never complains about a broken tab button. So it is save to merge this as it is.