Support media keys (volUp, volDown, mute) on Windows

[paulolc]

Media Keys support

Keyboards these days have dedicated media keys to mute, raise and lower the audio volume. It would be nice for Snap.Net to support them and raise/lower/mute the Snapcast clients volume.

Configuration

It should by default control the general snapcast volume (all the snapcast clients) but it could be configurable to just control one of the volume groups so that, for instance, I could define only to control the volume of the device of the house room I'm at.

[xenzh]

Now that there's controlScript support (and given that there's a multimedia widget in the iOS client already), it'd be great to

1. Have multimedia key support along with standard windows media overlay bound to snapserver control script actions.
2. For the iOS client, bind media widget's play/pause and volume controls directly to control script actions and add next/previous track bindings in case snapserver stream has control script configured.

This will allow media playback to be controlled via things like wireless headsets, which sounds especially great for mobile clients (i.e. now the mpd/snapcast playback use case is really awkward on iOS, in a sense that the media widget does anything but what you'd expect from any other media app).