Improvements to \offset

Improvements to \offset

Property can be specified with or without #' prefix.

Directed tweaks are possible.

Music is returned in case of a failed tweak on that music.  This eliminates
cryptic "post-event expected" error.

Regtest:
--remove #' prefix before property names
--remove unnecessary # before numbers
--add example showing directed tweaks
--faulty tweak converted to an override of correct property

Revise docstring of find-value-to-offset in `scm/music-functions.scm.'