Correctly handle changing canPlay, canPause, canGoNext and canGoPrevious

Summary:
Gwenview is adding MPRIS support in D10972. I tested it and noticed that it was quite buggy. Turns out,
Gwenviews implementation is (almost) fine, the problem was on our side. Unlike pure music players like
Spotify Gwenview changes canPlay, canPause, canGoNext and canGoPrevious when needed, but those updates
didn't reach our UI. The packages containing those updates were not parsed because they didn't contain
several fields.

Test Plan: Controlling Spotify and Gwenview (patched to support playPause) works fine now.

Reviewers: KDE Connect, mtijink

Reviewed By: KDE Connect, mtijink

Subscribers: mtijink, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D10981