refactor(gui): call the editing scope's key what it is

EditingScope.device_key is a String holding the persistent config key the open per-app profile is stored under: it is filled from, and only ever compared with, the `persistent_key: Option<&str>` that every BindingState method takes for its config lookups. It is not a DeviceKey and should not look like one, so the field is persistent_key.