Add support for combining QFutures together

QFuture<int> pipeline1 = QtConcurrent::run([]() { return 1; });
QFuture<QString> pipeline2 = QtConcurrent::run([]() { return "Hello world"; });

//Proposed API to combine multiple futures together
QFuture<void> combinePipelines = QFuture::combine({pipeline1, pipeline2});

combinePipeline.then([pipeline1, pipeline2]() {
    //Should print out "Results: 1 Hello World"
    qDebug() << "Result:" << pipeline1.result() + pipeline2.result();
});