Add `QFutureWatcher::setFuture(const std::future<T>&)`

When working with non-Qt concurrent operations, std::futures are often returned. There is no direct way to watch these futures and call a `SLOT` when they complete.

It would be nice to be able to watch a `std::future` or create a `QFuture` from a `std::future` (`static QFuture QFuture::fromStdFuture(std::future&& future)` or similar).