Grace note spacing on multi-staves

Steps to reproduce bug
1. Create a multi-staff score
2. Create one whole note in each staff of the first bar
3. Add two grace notes to the note in the first staff
4. Add two grace notes to the note in the second staff

Expected behavior: Grace notes should align vertically

Actual behavior: The first set of grace notes is further left than the second set

Workaround: Add the grace notes to each staff one at a time, instead of adding both grace notes to the first staff before going to the next.

MuseScore version: r.2507

(Operating System: Windows XP)

First reported by isaac: http://www.musescore.org/en/node/3916