Make QPromise thread safe but also ensure that continuation lambdas (then/fail/finally) are called in the thread of the promise instance they are attached to.
Make continuation methods const (then/fail/finally) and ensure that the resolved promise value/error is copied only when required, same for user lambdas (dispatching result is now fully handled by the PromiseData).
QFuture canceled with `QFuture::cancel()` now rejects attached promises with `QPromiseCanceledException`. In case the future is canceled because an exception (e) has been thrown, the promise is rejected with the same (e) exception (or `QUnhandledException` if not a subclass of `QException`).