status/bluetooth: Port to new GListModel API and track powered state differently
An alternative to !2055 (merged), this MR basically achieves the same goals (that is porting to new gnome-bluetooth API and fixing the "wedged" button state issue).
The main differences are:
- Uses a JS Set instead of an object to keep track of signal connections to connected bt devices
- keeping track of power state by only using the
default_adapter_powered
gnome-bluetooth API (that API factors in the airplane mode/rfkill state internally for us)