Fix colours of header bar for acitve and inactive windows

Summary:
Use and generate correct colours from the window manager palette rather
than the window palette in the headerbar.

BUG: 403426

This works beautifully on wayland, on X11 kwin doesn't send
NET_WM_STATE_FOCUSSED and GTK doesn't correctly WMs that don't support
it.

Might be worth not committing this till then.

Test Plan: Ran gtk3-demo which requests CSDs

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19603