[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: {F6960690}

Reviewers: #plasma, #vdg, ngraham, hein

Reviewed By: #plasma, #vdg, ngraham, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22230