Implement Undo/Redo functionality

[ExplosBlue]

Summary

This PR implements Undo/Redo support across the editor by introducing standardized document and selection models, and integrating undo commands into property editing workflows.

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Backend

Completed

• Implemented a unified document model
• Implemented a unified selection model
• Added atomic getters/setters for Emitter properties
• Added undo commands for Emitter properties
• Added undo commands for PTCL properties
• Added undo commands for EmitterSet property changes
• Added undo commands for copy & paste

Pending

• Add undo commands for adding/removing:
  • Emitters
  • EmitterSets
  • Textures

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UI

Completed

• Updated Emitter property inspectors to use undo commands
• Updated EmitterSet property inspectors to use undo commands
• Updated Field property inspectors to use undo commands
• Updated Fluctuation property inspectors to use undo commands
• Updated Child property inspectors to use undo commands
• Replaced existing editor widgets:
  • EmitterWidget
  • ChildEditorWidget
  • FieldEditorWidget
  • FluctuationEditorWidget
  • EmitterSetWidget
  • ProjectProperties
    with a unified inspector widget