Upgrade CTK to Qt6: Remove Deprecated Classes and Modernize Codebase

[chengyong]

This issue tracks the comprehensive upgrade of the CTK (Commontk) framework from Qt5 to Qt6, with a focus on eliminating deprecated classes, modernizing the codebase, and leveraging Qt6's improved APIs and C++ features.

Qt6 introduces significant improvements over Qt5, including:

1. Cleaner API: Removal of deprecated classes and functions
2. Better Performance: Optimized implementations and reduced memory footprint
3. Modern C++ Support: Leverages C++17/C++20 features
4. Long-term Support: Qt5 is in reduced support mode; Qt6 is the recommended version for new development
5. Improved Type Safety: Better compile-time checks and reduced runtime errors