Simplify coverage plots

[donkirkby]

The detailed view of the coverage plots is very cluttered. One option is described in #422, converting them to SVG and scripting the ability to show and hide some features. Another option is to simplify the static chart.

• Remove V3 overlap, since we now pass V3 reads through the G2P pipeline and the remapping pipeline.
• Move legend outside plot, or hold it separately. (Might be easier in SVG format.)
• Convert deletions, stop codons, partial deletions, and insertions from lines to markers.
• If it's still too busy, consider splitting into separate plots, as shown in Rougier.

[CBeelen]

More ideas:

• Make the plots wider for longer regions, to prevent all the markers being squished together.
• The grey bar that indicates key positions (?) could be part of the axes instead of being displayed within the plot.