[TaskManager] Follow KDE title & subtitle style
Summary:
We're currently using a custom style for song titles and artist info in tooltips.
This patch applies the common KDE title & subtitle style as seen in e.g. Kickoff or plasma-nm applet.
Test Plan:
Reviewers: Plasma, VDG, ngraham, hein
Reviewed By: Plasma, VDG, ngraham, hein
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D22230