Add 'Twilight' Global Theme
This MR adds a 'Twilight' Global Theme to the default set of Global Themes.
What is Breeze Twilight? Breeze Twilight is a variant of Breeze Default that opts for the dark variant of the Plasma Style instead. That's it.
With Kubuntu having taken this approach, and with Fedora now considering this approach as well, it's time we consider doing it upstream, because honestly Breeze Light for Applications + Breeze Dark for Desktop is a really nice looking combination.
Requires frameworks/plasma-framework!124 (merged) or a different fix that makes the PlasmoidHeader look how it's intended to look in this Global Theme. (if it's merged before then, it SHOULD be fixed before everyone sees this MR in effect in the actual release of 5.21)