Add a 'Legend' button that quickly explains what colors and checkboxes mean

[Infernio]

xEdit has something similar for its color coding and it's super useful as a quick reference if you're learning the tool for the first time. Or even if you already know all of that but just forgot an obscure combination.

Should probably behave the same way as xEdit's too, meaning that it always stays on top and updates if you click on something - so if I click on a Filter plugin that's missing masters, it would select that combination in the legend.