Draft: Replace "use accent color for titlebars" with "tint titlebars with accent color"

I need help! This replaces the previous "use accent color for titlebars" option with "tint titlebars with accent color", which we'd like to use out-of-the-box for Plasma 6. However, it would be better to also keep the previous option, and have something like:

Color headerbar with:
( ) Normal color
( ) Normal color but tinted with accent color
( ) Accent color

However, I couldn't figure out how to switch from a true/false state to three possible options; I'm quite unfamiliar with the code used here. Is there someone who could help with that side of things?

Resolves plasma-desktop#78