Make nested QtConcurrent::run() calls possible without the risk of a deadlock

Make nested QtConcurrent::run() calls possible without the risk of a deadlock.

Suggestions on how to implement this from the original reporter:

Solution A: Make the QThreadPool selectable.

If the thread pool is selectable, the main tasks can use a different thread pool as the subtasks. This avoids deadlocks and solves many other problems in complex applications.

Solution B: Detect nested calls and make them sequential.

A nested call of QtConcurrent::run just stops the current task and uses this thread to execute the nested task. In other words: If there aren't enough threads left to run all tasks in parallel, it automatically runs them nested sequentially to avoid deadlocks.