QFuture should be able to be completed with a known value

When setting up async pipeline, sometimes the results of the QFuture are already known. Having a completed future allows users to reuse the existing async pipeline. completed() can be implemented as a static function inside of QFuture to create a completed QFuture(). This prevents users from modifying the value of a QFuture return by QtConcurrent::run() or other similar functions. For example:

QFuture<int> myFuture = QFuture<int>::completed(5);
myFuture.then([](int printVar) {
     //This should run right away because myFuture is completed()
     qDebug() << printVar;
}

QFuture::completed() should be easy to implement with a QPromise, but I'm not sure if there's lighter weight solution inside of QFuture. 

An example implementation of this is located from lines 1742 to 1765 of h[https://github.com/vpicaver/asyncfuture/blob/master/asyncfuture.h|asyncfuture.h]. Here's the implementation:

template <typename T> 
QFuture<T> completed(const T &val) {   
    QFutureInterface<T> fi;   
    fi.setProgressRange(0, 1);   
    fi.reportFinished(&val);   
    return QFuture<T>(&fi);
}

QFuture<T>::completed() should also support QList<T> for setting multiple results.