Provide an API for starting an asynchronous task ​

We currently have QtConcurrent::run for starting asynchronous tasks, which doesn’t fit well with the rest of QtConcurrent APIs. It would be good to have something in Qt Core instead. So we can move QtConcurrent::run to Qt Core and also consider renaming it to qAsync.

Additionally, it will require some improvements. In particular: modernizing, using variadic arguments instead of overloads with different number of template args, making more flexible. For example, take a look at qtcreator's runAsync.