D10991: [mpris2 dataengine] Fix media key handling to match MediaPlayer2.Player

[Friedrich W . H . Kossebau]

kossebau added a dependent revision: D10972: [RFC] Exposing slideshow to MPRIS 
controllers.

REPOSITORY
  R120 Plasma Workspace

REVISION DETAIL
  https://phabricator.kde.org/D10991

To: kossebau, broulik
Cc: plasma-devel, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, 
sebas, apol, mart