Improve and extend the table of contents panel

This MR improves worksheet navigation, structure handling, plot integration, and general worksheet usability.

New features and improvements

• Added a Table of Contents panel showing worksheet entries as a hierarchical tree.
• Added support for Chapter, Subchapter, Section, Subsection, Paragraph, and Subparagraph entries.
• Added optional Command and Plot entries to the Table of Contents.
• Added direct navigation to headings, commands, and individual plot results.
• Added automatic synchronization between the active worksheet entry and the current Table of Contents item.
• Added search and filtering for headings, commands, custom titles, and plots.
• Added configurable visibility for hierarchy entries, commands, and plots.
• Added actions to expand or collapse the complete Table of Contents.
• Added renaming of headings, commands, and plots.
• Added actions to promote or demote hierarchy entries.
• Added separate actions for deleting a heading or its complete section.
• Added keyboard support for navigation, renaming, deletion, searching, and context menus.
• Preserved expansion and selection states when filtering, updating, or switching worksheets.
• Added stable identifiers and persistent custom titles for commands and results.
• Added a dedicated Table of Contents settings page for visible items and hierarchy fonts.
• Added compact font selection using KFontRequester.
• Updated hierarchy entry colors automatically when the application theme changes.
• Added Ctrl+Up and Ctrl+Down for moving worksheet entries.
• Improved moving multiple selected entries and complete hierarchy sections.
• Improved drag-and-drop with insertion previews, mouse-wheel scrolling, edge auto-scrolling, and cancellation with Escape.
• Improved focus and automatic scrolling when navigating between worksheet entries.
• Improved keyboard handling in read-only worksheets.
• Fixed incomplete hierarchy entry borders during drag operations.
• Improved Maxima plot loading when QFileSystemWatcher is triggered before gnuplot finishes writing the image.
• Refactored worksheet hierarchy handling into a dedicated WorksheetHierarchyManager.