[PATCH] PartCombine: Store the part-combine state in the engraver

[Reinhold Kainhofer]

Please review: 
http://codereview.appspot.com/3334043

So far, the Part_combine_engraver was very dumb: It simply created a text when 
a change event was encountered, but did not store the current state...

This patch makes it keep track of the currently active part-combine 
text/state, so that it will (sometime in the future) be able to e.g. repeat
the text after a line break, or to do other state-dependent things.

This patch does not change any behavior.

Cheers,
Reinhold
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Reinhold Kainhofer, address@hidden, http://reinhold.kainhofer.com/
 * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
 * http://www.fam.tuwien.ac.at/, DVR: 0005886
 * LilyPond, Music typesetting, http://www.lilypond.org