Make setSelection() work with missing points.

If the points array is missing points, for instance because the data
handler omitted them, setSelection() will now still properly find
the points that are being selected by looking at the points' idx
property instead of just indexing into the points array.
I find this a little frightening:

- The call to `this.getLeftBoundary_(setIdx)` has disappeared. Can you explain why it's no longer necessary?
- A direct lookup has turned into a loop over all the points.

Is there any way to make this fast by default in the (very) common case that you're not specifying your own DataHandler?

Also, this would greatly benefit from a test using your custom DataHandler. I can very easily imagine your use case breaking in the future without one.