Use more conventional KDE-style header/subtitle appearance

Typically KDE-style headers and subtitles use a normal font weight for the header, and a reduced opacity for the subtitle. This is both a stylistic decision (some of KDE's designers are not that keen on bold text), and also a practical workaround for the fact that you can't control what variants a font has. In particular using light and semibold weights is risky because many fonts do not have these variants, or distros may not ship them.

This patch aligns the grid and list delegates to use the more KDE-ish style and move towards the mockups at https://phabricator.kde.org/T12372. If necessary, we could also use Kirigami Headings for the heading label to increase the size a bit in an officially-supported way.