add remote soundcard volume control

[idelsink]

Originally reported by: Menno van der Graaf (Bitbucket: mercotui, GitHub: mercotui)

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A feature could be implemented where the Website could denote how loud the clients audiocard has to be set (using something like ALSA mixer). This technique was already used to set the volume, but was replaced by an pulse_audio based solution. Which frees it up for this use.

The benefit would be that you can easily equalize the volumes across different types of hardware.

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• Bitbucket: https://bitbucket.org/han_iot/dynamic-network-of-speakers/issue/8

[Mercotui]

The new audio library, as of commt 1de4d6e, no longer overrides the clients volume, so this might not be very relevant anymore, as you can configre your client volume yourself (i.e. by calling asla by hand)