value_str is a way to add text on a button without a separate label.
This saves having an extra label object, but uses more memory and is
removed in LVGL8
* add basic metronome app
* add bpb, tap to bpm, update widgets
* use event pressed for bpm tap
* move case statement break to the right place
* narrow bpm selection range, override touch events
* fix arc knob style
* re-enable sleeping in destructor
