Chord symbol layout

This is the second stage of chord symbol layout and spacing. The first stage (first commit) was to make each chord symbol contained fully within its own segment.
This second stage is to allow chord symbols to overlap subsequent segments that do not have their own chord symbol, using the same technique as rest[staffIdx].

The remaining problem is how to evenly space out the notes under a long chord name. See https://dl.dropbox.com/u/20727193/ChordSpacing.PNG for an extreme example. The mscx file is https://dl.dropbox.com/u/20727193/ChordTest.mscx

How to make the second crotchet in bar 2 spaced evenly? I think it might have something to do with springs in Measure::layoutX().