fix: Refactor undo/redo functionality and clean up related code

[KrishGaur1354]

This pull request significantly refactors and improves the undo/redo system in the application, making snapshot management more robust, modular, and consistent. The changes centralize snapshot logic, ensure undo/redo stacks are updated at the correct moments, and enhance the reliability of restoring and saving the application's state, including connections and compound selections.

Undo/Redo System Refactoring and Improvements:

• Introduced a new push_snapshot method in Graphics.py that encapsulates the logic for saving the current scene state, clearing the redo stack, and updating the undo/redo action states, replacing scattered snapshot code throughout the application.
• Updated the undo and redo methods in mainApp.py to use a more robust stack-based approach: undo now restores the previous snapshot and only pushes to redo if not at the initial state, while redo pushes the redone snapshot back to undo. Also, a new _update_undo_redo_actions method ensures UI buttons are enabled/disabled correctly.
• All actions that modify the scene (adding, deleting, or connecting items) now call push_snapshot at the appropriate time, ensuring that undo/redo stacks are always in sync with user actions. [1] [2] [3] [4] [5]

Snapshot Data Structure and Restore Enhancements:

• The snapshot structure in save_canvas now includes items, connections, and compound selections, and uses deep copies of objects to ensure accurate restoration. The load_canvas_from_snapshot method has been rewritten to fully rebuild the scene, connections, and restore class counters for unique naming.

Code Cleanup and Responsibility Clarification:

• The responsibility for pushing undo snapshots after scene modifications (such as adding or deleting unit operations) has been moved out of Container.py and into the main application logic, clarifying the separation of concerns and reducing redundancy. [1] [2]

Other Notable Adjustments:

• Removed redundant or now-unnecessary code for shortcut setup and initial undo/redo stack cleaning, as this is now handled by the new snapshot logic. [1] [2] [3]

These changes collectively make the undo/redo functionality more predictable, reliable, and easier to maintain.