applet: Simplify tracking of connected devices

According to my testing, none of this is needed. The BluezQt.Manager.devices list updates declaratively as needed, and the filter is gonna bind each one their connected state property.

It still results in a gazillion less changes being emitted in a crowded place during pairing, so a dedicated list property on BluezQt.Manager is still viable. See: frameworks/bluez-qt#1 (closed)