Tempo Segments

Is your feature request related to a problem? Please describe.
I'm always frustrated when I have to make a fast part

Describe the enhancement you'd like
Be able to add segments and change the tempo (would help with importing midi aswell)

Additional context

[Bentroen]

Hi! There's an experimental feature in NBS that does just this: rename a custom instrument to Tempo Changer (make sure to match caps), place it down and change the note pitch to change the tempo (for instance, setting the pitch to +300 will set the song to be 300 BPM at that point).

As this feature is unofficial, though, don't expect it to work anywhere else other than in the NBS player itself. It may be properly implemented sometime in the future. :)

(Related to #149)