Add low/high details visualisation modes for counter tracks

This patch adds support for switching between *low details* and *high details* visualisation modes of counter tracks using the "h" key (low details by default). The following two figures show the same track (with the same selection in the middle) rendered using the two modes:

*Low details:*
![countertrack_low](https://cloud.githubusercontent.com/assets/2546601/5229884/f494ff54-7715-11e4-8442-1f58021df8d1.png)

*High details:*
![countertrack_high](https://cloud.githubusercontent.com/assets/2546601/5229890/fc8248a2-7715-11e4-8e06-c525aa673122.png)

The following table shows the performance of the two modes compared to the original design (changed in #633):

![countertrack performance - google sheets](https://cloud.githubusercontent.com/assets/2546601/5229955/a635600a-7716-11e4-8917-16f43e516aeb.png)

Summary of the performance results:

* The low details mode performs slightly *better* (1% and 4%) than the original design *without* selection.
* The low details mode performs significantly *worse* (92% and 35%) than the original design *with* selection. If this is a problem, we can simplify the highlights (the low details mode currently uses the same circles as the high details mode).
* The high details mode performs significantly *worse* (12%-129%) than the original design *both* with and without selection (as expected). I think that this is fine.