Provide QFuture<void>::isResultReadyAt

Part of the usefulness of QFuture<void> is to store a QFuture<T> (with T being non-void) in a type-erased way.

We use it in this way, and found that QFuture<void> is missing a isResultReadyAt. Since that function doesn't use the T in any way, there doesn't seem to be anything to stop providing it.

QFutureWatcher<void> also provides the "resultsReadyAt" signal, therefore it would improve consistency aswell.