[applets/notifications] Use Header colors for notifications, if available
This brings the PlasmoidHeading used in the notification item into visual consistency with other PlasmoidHeadings, once we merge https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/98.
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):
And here's how the whole desktop will look once https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/98 is merged:
