Flag an error for isolated post-events occuring in music lists (issue 6929046)

[thomasmorley65]

Can't review parser-code, but from description:
LGTM

For the sake of easier understanding I'd have given these
examples,though, to show that printing yes/no is more a matter of
accident:

% fingering is printed
%\displayMusic
{ \tweak #'color #red -3 c }

% fingering is not printed      
%\displayMusic
{ c \tweak #'color #red -3 }



https://codereview.appspot.com/6929046/