Visually redesigned the counter track class

This patch improves the visual design of the counter track class. Rather than being displayed as a sequence of simple blocks, it looks like a line plot now:
![line_counter](https://cloud.githubusercontent.com/assets/2546601/4560559/f9b85c08-4ef5-11e4-8096-d20ded2f1e3f.png)
Note that this change completely fixes #625 (for counter tracks) as we draw the plot manually in view coordinates (to ensure constant line width) and thereby avoid using canvas transforms.