Add support for automatic unwrapping QFuture inside of .then()

auto download = [](QUrl url) {
   ...
   return QList<Tile>({tile1, ..., tileX});
}

auto processTiles = [](QList<Tile> tiles) {
   QFuture<QImage> future = QtConcurrent::mappedReduced(tiles, 
                                             processTile, //Map function
                                             reduceTiles) //Reduce function
   return future;
} 

auto saveImage = [](QImage image) {
   image.save(...);
}

auto future = QtConcurrent::run(download, url)
            .then(processTiles)
            .then(saveImage)