[dataengine/mpris2] Add global shortcuts for media control

The multiplexer registers global shortcuts for:
* play/pause
* next
* previous
* stop

and delegates them to the active player if triggered.

This could mean that media shortcuts for non mpris2 enabled
applications might break. The advantage is that we have global
shortcuts for media control.

BUG: 341587
FIXED-IN: 5.3.0
REVIEW: 122437