The name default.sf3 is no longer required, the soundfont name is resolved
directly so use the original name. It is TimGM6mb derivative with just
piano downloaded from:
<https://musical-artifacts.com/artifacts/2642>
This simplifies things a bit at the expense that the soundfont is always
included/installed (91 kB).
Added get_data_path() function for generic handling the path to common
data (eg. /usr/local/share/ultragrid if installed). The idea is to catch
all cases - installed (run with absolute or relative path) or run from
inside the source directory in one place.
