Add better hit testing for beats

[infojunkie]

When the user touches the sheet, we want to position the cursor at the nearest beat.

Right now, we only detect direct hits on the notes SVG elements, but these take up a very small percentage of the surface of their beat, which is a rectangular slice of a staff or system of staves.

We need to compute these beat rectangles and store them for fast query, just like we do for measure timestamp maps.