Add title, artist and album to MPRIS network packets

Summary: This diff adds the title, artist and album to the MPRIS network packets. That's useful when you need more detail than just "artist - title", for example in the future media control notification. It also fixes weird song descriptions for empty artist strings (e.g. Spotify uses an empty (but present) artist when playing ads)

Reviewers: KDE Connect, apol

Reviewed By: KDE Connect, apol

Subscribers: nicolasfella, apol

Differential Revision: https://phabricator.kde.org/D8957