GenomeView 1600: bugs, convenience and performance

Based on user feedback we have been able to fix another batch of bugs as well as introduce a number of convenient new features. Keep the reports coming, we appreciate all feedback.

Selected changes
GenomeView now remembers in which order tracks are, and will no longer jumble them if you switch chromosomes or when you restart GenomeView. It also remembers which track you made invisible.

The pileup and formats are now also loaded using background threads, which drastically improves the responsiveness of the GUI.

Fixed a recently introduced bug that made SNPs invisible.

Did some profiling and made a bunch of performance improvements.