[applets/taskmanager] Add a bit more padding around album/artist info in tooltip

Summary: Last visual change required to complete {T12640}.

Test Plan:
Before: {F8095898}
After: {F8096590}

Reviewers: #vdg, manueljlin, filipf

Reviewed By: #vdg, filipf

Subscribers: ndavis, filipf, plasma-devel

Tags: #plasma

Maniphest Tasks: T12640

Differential Revision: https://phabricator.kde.org/D27303