Issue 3012: Flag an error for isolated post-events occuring in music lists

Issue 3012: Flag an error for isolated post-events occuring in music lists

These can happen when writing things like

c \tweak #'color #red -3

since at the current point of time a tweaked post-event is not
syntactically recognized as a post-event without leading -, like

c -\tweak #'color #red -3

While this restriction will at some time be removed, in the mean time
we want to have this problem flagged.