Support changing colors from within a legend

[fedarko]

This would involve doing what Emperor does: rather than drawing just a colored square for each category we'd add a Spectrum.js colorpicker.

empress/empress/support_files/js/legend.js

Lines 145 to 150 in 5d19f0d

	// Add a color box (could totally be replaced by e.g. a Spectrum
	// color picker in the future).
	var colorCell = newRow.insertCell(-1);
	colorCell.classList.add("category-color");
	colorCell.classList.add("frozen-cell");
	colorCell.setAttribute("style", "background: " + info[key] + ";");

Actually adding the colorpickers likely wouldn't be too bad (although if a field has like a thousand groups it might slow down the page, see e.g. #344, so we may want to only create a colorpicker when a field is clicked on by the user) -- I think the hard part will likely be refactoring the code to use the selected colors efficiently? Like, preferably we'd store the Colorer created for a given field and just modify that, rather than calling colorByFeatureMetadata / colorBySampleCat again. Or maybe that wouldn't be too slow, I dunno.

This would impact:

• sample metadata coloring
• feature metadata coloring
• barplots (categorical feature metadata / stacked sample metadata)
• animations (I think colors are assigned based on the trajectory? We could probably draw a legend for the trajectory in the side panel, similarly to how barplot legends are drawn, so that users can manipulate it there.)