Bluetooth applet is showing on desktop without Bluetooth support

[antoniprzybylik]

The Bluetooth applet is showing on my desktop by default, but it only makes sense on devices with Bluetooth support, and this is unwanted behaviour.

[antoniprzybylik]

I think that the applet can't simply exit when no Bluetooth adapter is present, as it then wouldn't reappear if the user plugs in a USB dongle later. I think that this feature needs an applet-hiding mechanism in libcosmic (libcosmic#670).