Allow held-over or implied repeated notes

[chfin]

When a voice doesn't move between two stages, it might be held over to the second stage or even be missing (but implied) in the second stage. To account for these cases:

• include a reduced version of the second stage in the input of the horizontal pass (i.e. run vertical skipgram passes for reduced stage versions)
• adapt the prefix checker in the horizontal pass accordingly
• adapt the overlap rules to ignore held over notes when the second stage is reduced

[chfin]

We allow this kind of annotation now in the annotation app.