Audio rework: Drop phonon, replace with gstreamer

As phonon-gstreamer is no longer available on Qt6, and phonon-vlc lacks many features and seems to have various small issues that would need to be ironed out, I decided to try to swap completely to plain gstreamer. New audio backend code mostly based on phonon-gstreamer.

Current state: Playing local files, streams, remote Magnatune tracks, CDs and audio files with cue files seems to work. ReplayGain works.

Not yet implemented: Analyzer. Equalizer. Fadeouts (and crossfades?)

Fixes: #11 (closed)