[KMediaSession] Implement dynamic audio backend framework

KMediaSession is an audio player library that has an API which is close
to QMediaPlayer and which allows to use --- and dynamically switch
between --- different audio backends. At this moment there is
implementation for libVLC, gstreamer and QtMultimedia. Only QtMultimedia
is a hard dependency in order to at least have one functional backend on
all platforms; all other dependencies are optional.

KMediaSession has out-of-the-box, built-in MPRIS2 support, sleep inhibit,
and basic metadata support.

BUG: 462358

Closes #35