Add DeviceMute widget action io.element.device_mute.

[toger5]

This allows to send mute requests ("toWidget") and get the current mute state as a response.
And it will update the client about each change of mute states.

[dbkr]

I might need an explanation on what this is doing: whenever the audio or video mute state changes, we add a new listener for the device mute widget action? The listener presumably also fires (once?) to set the mute states and reply?

[toger5]

To my understanding the audio and video elements are not changing instead they mutate. So whenever they are set we connect the listener and when they are unset we remove it.

[dbkr]

I'm not sure it's that simple. Why not just use a callback and then the problem goes away? Also it will make the code less nested.

[toger5]

I updated this. So it is no creating the useCallback and recomputes the on/off when the callback changes.

[dbkr]

In general, comments would be helpful: I say this as someone who's knowledge of this codebase is now a little rusty :)

[dbkr]

Looks good as far as I can see, comments make it much clearer, thanks!

[fkwp]

lgtm