Always use white color for headerbar text

Summary:
Elisa uses the color scheme's highlighted text color for headerbar labels. This is not
semantically correct since this color relies on being above the color scheme's highlight
color to ensure adequate contrast. However the headerbar's blur does not do this; rather,
it always produces a dark-ish color. This causes issues for color schemes where the
highlight is light and the highlightext text color is dark, such as Materia Light.

This patch fixes the problem by always using a hardcoded white color, which always looks
good against the dark-ish blurry headerbar.

BUG: 410560

Test Plan:
Now looks good with the previously problematic Materia Light color scheme:

Worst-case scenario for contrast with white album art; headerbar still darkens it enough to make the text easily visible:

Reviewers: VDG, Elisa, mgallien, astippich

Reviewed By: Elisa, mgallien

Differential Revision: https://phabricator.kde.org/D22949