QFuture does not allow to access a move-only object via resultAt()/result()

If an object of move-only type is used in pair with QFuture, we cannot use simple APIs to access the stored value such as result() or resultAt(). Example:

QFutureInterface<std::unique_ptr<int>> iface;
iface.reportResult(std::make_unique<int>(42));  // add 42 ptr to int into storage
iface.resultReference(0);  // OK, returns const T &
auto future = iface.future();
future.resultAt(0);  // Error, returns T instead of const T &

This happens despite the fact that resultAt() directly calls resultReference(), so we just lose a reference there (and also forced to copy an object).

It is unclear, why we always return T instead of const T & (as opposed to QFutureInterface function) - there's no detachment or anything in between.

Anyhow, I found a single possibility to get the results: using QFuture::const_iterator as they always return references (which is clumsy to use).

—

The issue exists in prior versions (e.g. 5.15) but unlikely we can change the APIs there, so Qt6 is the best bet.