Simplify MPRIS callback code

Summary:
Makes sure the MPRIS data is always up-to-date, so requesting updates
separately is never necessary. Also provides a way to remove callbacks.

Test Plan: All information still appears in the MPRIS activity and the controls still work.

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Differential Revision: https://phabricator.kde.org/D9084