Added switching between audio outputs

Summary

Adds missing functionality of system volume to select current audio output of the connected remote device. This requires changes in kdeconnect-kde to be made to make it work. Basically, patch just adds radio button for every audio device in UI, reading the additional field from network packet indicating whether audio device is currently set as default and sending data about selected device on user input to connected device in plugin backend.

As for now, I'm not sure whether the radio butons should be hidden if we didn't receive data about current device from remote (i.e. remote kdeconnect implementation is not updated yet)

Requires kdeconnect-kde MR: kdeconnect-kde!364 (merged)

Test Plan

UI is similar to what we have in Plasma. Click on radio button or on name of audio device to select it

Toast on label long click SCR_20210117_154739