Use Header colors in PlasmoidHeading headers

This brings applets with distinct heading areas into visual consistency with windows, once the Tools Area MR lands.

When using a color scheme that lacks Header colors, there is no visual change whatsoever. But here's how it looks when you are using a color scheme with header colors (note: this is a WIP color scheme and the final shade of gray may be different):

In Kickoff:

In the System Tray grid:

In Plasma-nm:

In the clock:

The full effect:

cc @teams/vdg