Add option to show a Sashimi plot based on alignments

If enableSashimi="true" is set, Sashimi menu option will appear on alignment popup. User will then be able to select the alignment tracks and gene track to display. A separate window will appear, displaying a Sashimi (see http://genes.mit.edu/burgelab/miso/docs/sashimi.html) style plot of splice junctions. The thickness of the lines is based on the junction depth (log scale), and can be filtered by minimum. By default the minimum junction depth is what the user has in the settings, this can be changed with popup dialog.