Provide "fluent interface" for QtConcurrent::run

QtConcurrent::run provides an API for running a callable object in a separate thread. This functionality is represented by a few functions with different signatures. One of the functions allows a user to specify a thread pool on which the task should be executed.

There are a few requests from Qt users regarding adding more parameters to the API. For example, set a priority for a task.

Adding more parameters leads to increasing API complexity. Furthermore, some of parameters are not mandatory. For example, it should be possible to set a priority for a task, but not specify a thread pool.

In light of this, it would be nice to have a fluent interface for adjusting some related parameters. It should also improve code readability.

Example of a such interface (all calls between "task" and "spawn" are optional):

QtConcurrent::task(doSmth)
    .withArguments(1, 2, 3)
    .onThreadPool(pool)
    .withPriority(10)
    .spawn() // Returns a future object
    .waitForFinished();