Opacity of tracks based on a score/mappability

[jmonlong]

This was an idea to make it easier to distinguish separate tracks (with different assigned color palettes) and still visualize the mapq information by making the reads more transparent the lower the mapq. Right now, if we color by mapping quality, all the tracks get colored and we lose which path belong to which track.

I'm also using this option for "coverage" tracks to represent coverage level: the more opaque the higher the coverage.

Just a suggestion at this point. This was easy to add. Another harder option could be to provide a way for the user to define color palettes and/or color gradients for mapping quality.