bluetooth: Show the Bluetooth menu when there were setup devices

If we detected that Bluetooth devices were setup, show the Bluetooth
menu so that users can easily turn Bluetooth back on.

This is a bit of a hack, as we cannot detect whether there is a
Bluetooth adapter at all when it's disabled, so we cannot tell whether
there were any Bluetooth devices setup, at some point. This state is
saved in the gnome-shell GSettings in the had-bluetooth-devices-setup
key.

Checking whether we saw Bluetooth devices at one point is a good
enough guess of whether there will be some in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=723848