Draft: 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 works. TODO: Volume, mute etc are not updated if changed outside Amarok. There might also be miscellaneous crashes and deadlocks. Streams, CDs etc probably don't yet work. ReplayGain. Analyzer. Visualizer. Fadeouts (and crossfades?)

Fixes: #11