Add Default PandasTableWidget Delegate

I removed the hardcoded text alignment from the model and defined an AlignCenterDelegate in the view. To combine center alignment and input validation (to prevent crashes), I created a DefaultDelegate that combines InputValidationDelegate and AlignCenterDelegate. PandasTableWidget now uses this DefaultDelegate but it can be subclassed and modified to create new delegate functionalities.

Other changes:

• Refactored CheckboxCenteringDelegate and BooleanPaintingDelegate to use AlignCenterDelegate.
• Added a detailed example of subclassing DefaultDelegate.
• Added a simple example of DefaultDelegate subclassing.
• Updated docstring to include details on PandasTableWidget and DefaultDelegate.