PR: Rework taskmanager thread management

[jnsebgosselin]

This pull request refactors and improves the thread management logic in the QtAppUtils task manager. The changes focus on making the thread lifecycle more robust and predictable. Specifically:

• The worker thread is now managed so it persists for the entire lifetime of the task manager object, rather than being started and stopped for each batch of tasks.
• The shutdown logic has been improved. The close() method now ensures that all running tasks are completed before the worker thread is quit and joined, reducing the risk of premature thread termination or resource leaks.
• Superfluous code related to thread and task state handling has been removed or simplified, resulting in a cleaner and more maintainable implementation.
• These changes make the task manager more efficient and reliable, particularly in scenarios involving multiple batches of tasks or application shutdown.