DoubleTime 3.0

A port of the doubletime (https://musescore.org/en/project/doubletime) plugin, for Musescore 3.0.

Copies all notes in the selection to a new score, with their durations doubled.

Known Issues:
Doesn't set the key signature of the new score, so accidentals can get messed up (but it should always produces equivalent notes e.g. A# instead of Bb)
Doesn't copy ties between notes