rfkill: Some fixes and cleanups

• turn RfkillManager into a GObject
• respect HasAirplaneMode property

This enables code like the following:

const RfkillToggle = GObject.registerClass(
class RfkillToggle extends QuickToggle {
    _init() {
        super._init({
            label: _('Airplane Mode'),
            iconName: 'airplane-mode-symbolic',
        });

        this._manager = getRfkillManager();
        this._manager.bind_property('show-airplane-mode',
            this, 'visible',
            GObject.BindingFlags.SYNC_CREATE);
        this._manager.bind_property('airplane-mode',
            this, 'checked',
            GObject.BindingFlags.SYNC_CREATE);

        this.connect('clicked',
            () => (this._manager.airplaneMode = !this._manager.airplaneMode));
    }
});