Old-style multimeasure rests

[adrianholovaty]

We should add support for encoding the "old-style" of multimeasure rests, as pointed out in this comment by @clnoel.

More precisely, this is the presentational decision that multimeasure rests should use a combination of whole and double-whole rests instead of the contemporary "I-beam".

It would also be good to encode the threshold at which a rendering engine should use an I-beam instead of individual rests. For example, see the "To display symbols for multimeasure rests" section in this Finale help page.

An obvious place for this information would be in the <score>'s <multimeasure-rests> element — i.e., scorewide as opposed to defined in each individual <multimeasure-rest>. Perhaps we encourage the use of a scorewide encoding but allow overriding for any individual <multimeasure-rest> element?

[rettinghaus]

In printed orchestral parts these old-style rests usually were used for up to 10 measures, more were indicated with the H-bar rest. So for the scorewide encoding it should be a threshold indicating the switch between the two styles.