improve bpm so that change in bpm doesn't result in rhythm hiccup...

[davidedc]

if the bpm is changed, there is a slight hiccup in the rhythm.

This is because when bpm is changed, the timer that generated the old pace is destroyed, and a new one is created from scratch with the new rhythm. The problem being that the new timer starts from an absolute arbitrary time when the program has changed, so it has no alignment whatsoever to the old pace.

It should be smarter than that: the pace should be updated on the first beat of the old pace, so it is aligned with the "old" beat.