fix #22473: alignment of breves, layout with mixed notehead size

The basic change is to center whole notes and breves using *chord* offsets rather than *notehead* offsets.  This works better, but had a bit of a ripple effect, resulting in a larger diff than you might expect to fix such an obscure bug.  On the very positive side, however, a number of other bugs got fixed more or less for free as a result of these changes: the ones listed in the commit log, at least one other from the tracker (http://musescore.org/node/22883), and a couple of others I found as I went that were not in the tracker and did not seem worth adding.  The new vtests show the results.