improve autosave reliability

[KrabCode]

clicking the square STOP button in PDE does not save the json gui state to disk

maybe using the dispose() function as described here would be more reliable?

[KrabCode]

it does not improve the behavior nor does it worsen it, but the code looks much better now, so let's keep the dispose() approach

this new implementation looks a lot easier to read:

    public static void registerAutosaveOnExitHandler() {
        GlobalReferences.app.registerMethod("dispose", singletonInstance);
    }

    public void dispose(){
        createNewAutosave();
    }

than this:

public static void registerExitHandler() {
        Runtime.getRuntime().addShutdownHook(new Thread(JsonSaveStore::createNewAutosave));
}