Allow property paths as scalars and in assignments

Allow property paths as scalars and in assignments

They cannot be embedded LilyPond since #{ 2 . 2 #} already is valid
music.

Property paths must have at least two syntactic elements to be
recognized as such and must not start with a Scheme expression.
Technically, 3 . #'() counts.

This affects overrides and sets as well.


Also contains commits:

Run scripts/auxiliar/update-with-convert-ly.sh


convert-ly rule for nicer beatStructure syntax