SVG export

[jrobinso]

Add a button to export the SVG to a file, as we do in igv.js. I've requested an API to get the SVG from the circ view component here: GMOD/jbrowse-components#2610. However, in the absence of an API we should be able to easily get the SVG with a DOM query, there's only 1 SVG element in the circ view. Once we have that I assume we can export it.

DOM looks like this:

[turner]

I've been looking at this. It would be relatively straightforward except the svg element is parented by a div has transforms applied to it - a 90 deg. rotate around a translated origin - that will need to be included in the svg export. Not clear what the best way to do that is. Ideally all transforms should be only in the svg.