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 division in stafftext.cpp should never set the *style* setting for swingUnit.  The result of the division is written to the score file for individual stafftexts only, not for the style.  So to the extent division might cause problems, it's possible that the file might contain swingUnit values of 0.4999 0.5001 or whatever for individual stafftexts, but never for the style setting.  That could probably be fixed by forcing the value written to be rounded to two decimal places - I have no idea how the xml.tagE code would deal with that.

Now if 0.5 and 0.25 cannot be represented exactly even when assigned directly, then indeed we have a possible problem with the style settings as well.  I kind of doubt any implementation would have a problem with it, but I guess you never know, unless maybe it's specified in the standard.