Swing Playback implementation using staff texts

Swing playback using staff texts implemented, facilitating local control over swing parameters. Also added tests for the same.
As per discussion on IRC, it's been decided to change to storing the strings "Eighth" and "Sixteenth".  So the type of the style parameter changes from double to string, with a simple "if" or "switch" statement in stafftext.cpp to convert that to ticks - eg, if (s == "Eighth") swingUnit = Division/2.  Similarly for the code in read() and write() - if (swingUnit == Division/2) s = "Eighth".

That of course means updating the tests again too.