Make the badge text readable on dark color schemes

Summary:
The background color should not be used for the foreground text
color, I believe. The background color is set to theme.highlightColor,
so use theme.highlightedTextColor for the foreground. This ensures
the text is readable on a dark color scheme.

Test Plan: unreadable text before, readable now

Reviewers: broulik, hein

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10697