[Lock screen] Follow KDE title & subtitle style
Summary:
We're currently using a custom tyle for song titles and artist info in the lock screen's media controls.
This patch applies common KDE title & subtitle style as seen in e.g. Kickoff or plasma-nm applet-
Because the lock screen has increased font sizes, corrections for the title and subtitle have also been made.
Test Plan:
Before:
After:
Reviewers: Plasma, VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D22231