Stream to output devices through sound mode selection

[albaintor]

This PR lets select one ore several output devices to stream audio to.
How it works : at connection, the list of available output devices are retrieved (airplay compatible)
The through the sound mode selection, a list of possible combinations is displayed :

1. First entry with the name of apple tv : disable all output devices except Apple TV
2. Next entries : list of possible combinations

For example if you have 2 homepods e.g "Homepod 1" and "Homepod 2" you will have :

1. Apple TV
2. Homepod 1 => stream audio to AppleTV + Homepod1
3. Homepod 2 => stream audio to AppleTV + Homepod1
4. Homepod 1, Homepod 2 stream audio to AppleTV + Homepod1 + Homepod2

The dropdown list should be improved in the webconfigurator because entries are cropped, otherwise it works perfectly.
I guess this is a temporary approach : the list can grow up fastly if you have many airplay devices. I limited the number of items between 1 (1 device enabled) and 5 (5 devices enabled)

[zehnm]

Sorry for the late review and thanks a lot for the audio output selection feature!

Please fix the duplicate output devices and the instance names. The remaining comments are cosmetic things and questions.

[zehnm]

I'll add a task in the web-configurator to better scale the sound mode dropdown list, and maybe even make it a free-text entry as for the input/app selection.

@martonborzak is there already a sound-mode selection feature in the media-player UI? I couldn't find it :-)

[albaintor]

I am testing modifications....

[albaintor]

Illustration of the dropdown too small. Possible to grow it up (max-width=XXX) ?

[zehnm]

Have you already looked into the outputdevices_update callback?
It looks like this could be used to update the currently selected outputs and propagate the sound_mode attribute to R2.
Quick test with deselecting a speaker in control center:

[albaintor]

Have you already looked into the outputdevices_update callback? It looks like this could be used to update the currently selected outputs and propagate the sound_mode attribute to R2. Quick test with deselecting a speaker in control center:

The callback was already there in your code but not used, I added the missing lines to notify the remote of updated output devices

[zehnm]

Looking good, thanks again!
The number of calls to self.output_devices could be reduced, but that's something we can fix with the upcoming speaker group feature.