Don't make Headings 20% transparent, to match Kirigami

Summary:
We recently removed Kirigami Headings' slight transparency, after concluding that it looked inappropriate to have Headings with lighter text than their textual content. PlasmaExtras.Heading does the same thing, so we should make the same change here, both based on the inherent merits of the change, and also to maintain consistency with Kirigami.

This also slightly improves matters for people who complain about poor text contrast throughout Plasma.

Test Plan:
Browsed System Settings; headings are now ever so slightly darker (or lighter, for users of Dark themes)

Also, here are some befores-and-afters for a widget I'm working on that uses Headings. Before:

After:

It's subtle; you'll need to flip between the two with the arrow keys to see the difference, and may need to also zoom in (meta++)

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10899