Implement configurable indent guides for LaTeX environment visualization

[Copilot]

Implements issue #4123 by adding configurable indent guides (vertical lines showing indentation levels) to the TeXstudio editor, similar to those found in VSCode and OverLeaf.

Features

• Visual Indentation Guides: Displays subtle dotted vertical lines at indentation levels to help visualize nested LaTeX environments
• Configurable: Can be enabled/disabled via Options → Configure TeXstudio → Editor → Show Indent Guides
• Default Off: Feature is disabled by default as requested
• LaTeX-Aware: Properly handles spaces, tabs, and mixed indentation common in LaTeX documents

Example

The feature helps visualize nested structures like:

\begin{itemize}
|   \item First item
|   \item Second item with nested content:
|   |   \begin{itemize}
|   |   |   \item Nested item 1
|   |   |   \item Nested item 2
|   |   \end{itemize}
|   \item Third item
\end{itemize}

Where | represents the subtle vertical guide lines that appear in the editor.

Implementation

This implementation addresses all reviewer feedback:

• Core Logic: Added ShowIndentGuides flag to QEditor and implemented drawIndentGuides() method
• Configuration: Added showIndentGuides option to LatexEditorViewConfig with proper UI binding
• Performance: Efficient algorithm that only processes visible lines and draws guides only on indented lines
• Visual Style: Uses subtle dotted lines with low alpha transparency to avoid distraction
• Drawing Integration: Guides are drawn within the document rendering context before text to appear behind content

Technical Details

The implementation follows existing TeXstudio patterns (similar to ShowPlaceholders feature):

• Integrates with the existing configuration management system
• Uses consistent font metrics without external dependencies
• Draws guides during the editor's paint event as part of document rendering
• Handles various indentation scenarios including tabs, spaces, and mixed indentation
• Only draws guides on lines that actually have indentation (skips unindented lines)

This enhancement makes it significantly easier to understand and edit complex nested LaTeX structures, addressing a commonly requested feature for improved code readability.

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