This adds several capabilities to the task registry:

• You can now a thread task, meaning you execute some lambda in a new thread and this lambda execution is a new task with its parent beeing the currently running task on the old thread.
• You can schedule a task on the scheduler. The task state is then 'Scheduled'. As soon as the scheduler executes the task, the state changes to 'Running'.
• A task can have a printer object. A user can define such an object, which needs to implement auto operator()() -> std::string. This method is executed as soon as a snapshot of a task is requested. This way, a string is only created when really needed.