indicate significance of marker genes

[alexvpickering]

This was a feature request/suggestion from Rashmi. She suggested e.g. 5 coloured circles beside each gene where the number of filled circles indicates significance (like a 5 star rating).

I tried something similar at one point (just showing the p-value beside the gene name) but abandoned it because p-values were always very small so it didn't seem useful. It would be worth exploring other measures (e.g. fraction of cells expressing marker above background as compared to any other cluster).

[alexvpickering]

This conflicts with #81 which necessitates shiny::selectizeInput server-side rendering for performance. Once this issue is resolved then it should be trivial to implement with shinyWidgets::pickerInput. Alternatively, this could just wait for web-app version with is more amenable to style customisation.

[alexvpickering]

Found this example with markup in the options that should make this doable.

[alexvpickering]

This was fixed by moving to a table based layout for selecting genes.