Adds Previous/Next MPRIS commands

CxOrgrequested to merge
cxorg/Shortwave:previous_next_mpris into main

This branch (previous_next_mpris) adds/extends MPRIS functionality to play next/previous items in the favourites list. The play/pause is now updated to work as expected from MPRIS client play/pause toggle. Compiles and runs on my local without issues or syslog warnings. I'm not sure the build pipeline failures are relevant as World controls release packaging.

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