Introduce ThreadService@getParent(Thread)

Due to the use of a thread pool, it is impossible to determine which thread called the current one.

Sometimes we need that, though, e.g. when ImageJ 1.x wants to determine the (thread-local) macro options and runs in a thread that was spawned through the thread service.

So let's add API to determine the parent thread, so that the ImageJ 1.x patcher can be extended to support macro options even when the `ThreadService` is involved.