Swing Playback implementation using staff texts

Swing playback using staff texts implemented, facilitating local control over swing parameters. Also added tests for the same.
The question is whether a 0.5 or a 0.25 have an exact representation in a double or qreal, and have that on all platforms MuseScore can run on.
Also it does get set via a division in stafftext.cpp, so can get wrong there already.