Forward an on_device_hotplugged to aout player callback

Currently audio device hotplug is detected in aout_HotplugNotify but there is no way to forward this to the interface. We should either add it to the aout player callbacks or add a new callback system for this.

What's blocking this is that we need to forward the id string and the name string to the user interface, so it can't be through a vlc_variable.

The previous Qt UI was updating the device menu each time it was opened to mitigate this.