Enhancement: Support external volume control

[mill1000]

I've been contemplating an enhancement to support external volume control. I have multiple renderers, and other sources, which I want to have a single point of volume control.

Currently the volume is passed to the output module here. The default (and only) module, gstreamer, implements a software volume control (via playbin). It doesn't appear possible to pass this control on to the underlying sink (ALSA, PulseAudio, etc).

gmrender-resurrect/src/output.c

Lines 202 to 207 in 2a4d744

	int output_set_volume(float value) {
	if (output_module && output_module->set_volume) {
	return output_module->set_volume(value);
	}
	return -1;
	}

I experimented with a build that clears the soft-volume GstPlayFlags but it does not fallback to the sink's volume. It simply disabled volume control

This idea was also discussed in #208, and an ALSA-only implementation was made in a fork here, here and here.

For obvious reasons, directly controlling an ALSA mixer is the wrong approach. Instead, I think we should either externally signal the change (D-Bus?) or call a hook. That way another utility could handle the volume control. (ALSA mixer, IR command, etc)