add suport for metric modulaton, incl. playback

Things like [dotterd qzarter note] = [quarter note]

See https://musescore.org/en/node/71061, https://musescore.org/en/node/94426, https://musescore.org/en/node/95196 and https://musescore.org/en/node/109981

See also https://en.wikipedia.org/wiki/Metric_modulation

These are sort of relative tempo texts, as they change the tempo relative to whatever it had been set before