Next job is to finish bar overlays.
- Do overlays live in LinePanel, ScorePanel or a new child of ScorePanel?
- Overlays must go away when mouse movement stops for a while or piano things happen.

The problem with overlays on the LinePanel is they could be better positioned by the ScorePanel (since it knows where other things live relative to the linePanel). Also, they are affected by the LinePanel's scaleTransform.

I'll put them in ScorePanel. All mouseover overlays should go away when the mouse leaves the panel.

Translating mouseMove point to linePanels. Transforms have a 'Point Transform(Point fromPoint)' method and an Inverse property (returning another Transform) which come in very handy.