[branch 1.2] Measure rest should be shown as breve if measure >= 4/2

Context: branch rev. 5360

This is halfway between a bug report and a feature request: it is better to show a whole measure rest as a breve if the measure length is greater or equal 4/2 (= 1 breve).

The attached patch back-ports, with adaptations and some simplifications, a similar patch for trunk, adding 4 code lines to function Rest::layout() (file rest.cpp, lines 340 and foll.). Tested in several scores with no observed negative side effects.

The patch does not manage a whole measure rest in a multi-voice context, but this was not managed before either.

Thanks,

M.