Fixed kdeconnectd deadlock on Windows caused by systemvolumeplugin

IAudioEndpointVolumeCallback::Release was called in callback functions of IMMNotificationClient (through the call to SystemvolumePlugin::sendSinkList), however https://docs.microsoft.com/en-us/windows/win32/api/mmdeviceapi/nn-mmdeviceapi-immnotificationclient points out that:
*To avoid dead locks, the client should never call IMMDeviceEnumerator::RegisterEndpointNotificationCallback or IMMDeviceEnumerator::UnregisterEndpointNotificationCallback in its implementation of IMMNotificationClient methods.
*The client should never release the final reference on an MMDevice API object during an event callback.

So I moved that part of code to another thread so it will successfully run after callback functions work out and call only if endpoint needs to be released (device was removed), so it won't spawn new thread for every generated event