PartCombine: part-combine texts on first real note rather than rests

PartCombine: part-combine texts on first real note rather than rests

-) If the context property partCombineTextsOnNote is set, the part-combine
   texts are not printed immediately if there is a rest or skip, but on
	 the next following note. If the voice contains a note, they are printed
	 immediately.
	 This is needed if one voice has a full measure rest and the other e.g.
	 r2 r4 c4.
-) The event triggering the part-combine text is cached in that case
   and the Part_combine_engraver now also listens to note events. The
	 texts are then created at the first moment when a note is encountered.