Implement basic concept for rendering labels on GWAS display

[cmdcolin]

This makes an arbitrary cutoff and draws features with score > 30 for demo purposes

I think that a systematic way to determine which feature to render a label on is needed. This should require literature and/or code cross reference to other GWAS libraries

In some cases, you might want to render nearby gene

In some cases you might want to render just the "leading SNP", which gets into the idea of linkage disequilibrium which has not been addressed much

If we have realistic datasets to help with this, it might help

CC @abcdw @pjotrp

Screenshot on demo data

The logic behind this PR is:

The 'renderer' returns a click map, then the click map is propagated to the 'display', and then the display renders 'highScoringFeatures' (with that score>30 cutoff) using HTML text labels that are absolutely positioned. could also use svg text nodes alternatively.

[cmdcolin]

(also, I just generally apologize that doing things like this is difficult in jbrowse. I wish it was as easy as a ggplot with adding a label layer but currently a lot of manual work...let me know if i can help with any questions)

[abcdw]

@cmdcolin Thank you very much for this prototype! I'll try it out soon and share the feedback and updates probably next week.