Add tempo slider to playback widget.

[yrammos]

For better or worse, many MEI/MusicXML files do not include tempo data, and the default tempo of the widget can be entirely inappropriate, sometimes to the point of incomprehensibility (e.g. Schubert's Gute Nacht from the corpus).

We should add a tempo adjustment control in the widget, defaulting to the tempo encoded within the score file, and falling back to an arbitrary value of no such value exists.

[yrammos]

(Originally discussed in #184.)

[meduzen]

I see the MEI spec has a <tempo> element mentioned at 36 places of the spec (according to “find in page” browser feature). Could it help in any way? If yes, is a tempo control still useful?

What would be the unit? BPM? Speed scale like on YouTube?

If BPM is the wanted unit, we need high precision, so I’d maybe go for a slider combined to steppers:

[pettter]

@meduzen The tempo element doesn't necessarily help much I think, at least not without quite a bit of coding (not something that's impossible to do, mind, just cumbersome), but your proposed design should work great as a start.

[yrammos]

I checked out the MEI docs and must agree with @pettter: the <tempo> element is too free-form to be usable without a bit of coding. Therefore the only easily implementable default can be an arbitrary value (e.g. drawn from the configuration file).

@meduzen I like your design, too. The value step can be 4bpm, in line with the step of many commercial metronomes.