Fix unproper nesting of various property overrides (issue 6685044)

[janek . lilypond]

LGTM

can you please change tenses in the commit message?  Every time i see a
sentence like "\acciaccatura and \slashedGrace lose any previous setting
of Flag.stroke-style." i think that it means that *the result of the
patch* is that previous settings are lost.
I think it would be better to write "before this patch, \acciaccatura
and \slashedGrace were losing any previous setting of
Flag.stroke-style".

cheers,
Janek

PS as usual, i dedicate my code reviews to Graham Percival. btw, i hope
that's not embarassing for him...

http://codereview.appspot.com/6685044/