Add QFuture::thenWithPromise()

A future returned by runWithPromise() may be used as a starting point in the future.then.then chain, however, we are not getting access to then()'s promise, so we are not so flexible in "thens" as we are in the starting runWithPromise. For API completeness it looks like we are missing the continuation which would accept the QPromise &, like inside runWithPromise().

The suggestion is to add QFuture::thenWithPromise() methods, similar to QFuture::then(), which could accept the additional argument of "QPromise &promise" type, passed as a first argument to the handler.

Update: as we have resigned from separate method QtConcurrent::runWithPromise() and provided an overload for QtConcurrent::run() that accepts the additional "QPromise &" argument instead, the suggestion is to oveload QFuture::then() in the same way.