Refactor AbstractImageRenamer to return a message

[ottlinger]

This would seriously improve testabllity and lead to a more clearer design since the renaming is not bound to the UI technology.

Code example:

ExecutorService pool = Executors.newFixedThreadPool(2); // creates a
pool of threads for the Future to draw from

Future<Integer> value = pool.submit(new Callable<Integer>() {
    @Override
    public Integer call() {return 2;}
});