Outliner Tree Element Refactoring

This project aims to port the remaining tree elements to the new object-oriented design of the Outliner codebase. The project will involve creating new subclasses, modifying existing code, thoroughly testing changes, and documenting the modifications made. The deliverables for this project include a detailed report and updated code of new AbstractTreeElement sub-classes representing the remaining tree elements. The project will contribute to the ongoing development efforts of the Outliner by providing a necessary refactoring that will make future development work easier and more efficient, and the Outliner module's documentation will be improved.