DeclarativeManager needs to implement connectedDevices list property

DeclarativeManager class — which is exposed as BluezQt.Manager to QML — provides devices list property, but downstream consumers such as plasma-mobile shell and Bluedevil applet also manually keep track of a filtered list which should contain only the connected devices. Being a pure QML/JS code, the implementations are ridiculously complicated, involving custom catch-all Connections:

    Connections {
        target: BluezQt.Manager

        function onDeviceAdded(): void {
            updateConnectedDevices();
        }
        function onDeviceRemoved(): void {
            updateConnectedDevices();
        }
        function onDeviceChanged(): void {
            updateConnectedDevices();
        }
        function onBluetoothBlockedChanged(): void {
            updateConnectedDevices();
        }
        function onBluetoothOperationalChanged(): void {
            updateConnectedDevices();
        }
    }

and manual shallow JS array comparison (which used an incorrect == operator relying on Device.toString() implementation — something which I fixed in plasma/bluedevil!171 (merged)).

Let's drop all that mess by introducing an in-house BluezQt.Manager.connectedDevices observable property.