Change Audio Module to explicit enable

[fel115]

https://github.com/ElektraInitiative/opensesame/pull/75#discussion_r1344060717: Somehow like this:

if audio/bell != /dev/null || audio/alarm != /dev/null {
    let audio_bell = config.get::<String>("audio/bell");
    let audio_alarm = config.get::<String>("audio/alarm");
    tasks.push(spawn(Audio::get_background_task(
        Audio::new(audio_bell, audio_alarm),
        audio_receiver,
        nextcloud_sender.clone(),
    )));
}

Or add a audio_enable to the elektra config.