Refactor: Standardize Project Layout and Create Runnable Modules

[VoxleOne]

Task

To improve maintainability and ease of use, the project structure should be standardized to follow modern Python best practices. This involves making the CLI and GUI runnable as Python modules.

Acceptance Criteria

• The command-line interface can be run using python -m xlabel.cli.
• The graphical user interface can be run using python -m xlabel.gui.
• The purpose and contents of the xlabel_gui/ directory are clarified and integrated into the main xlabel_gui.py entry point or a clear GUI package structure.
• A __init__.py file is present in the xlabel directory and its sub-packages (xlabel_format_converters, xlabel_gui) to define them as Python packages.

Rationale

A standardized, module-based structure makes the project easier to navigate, test, and package for distribution. It is a foundational step for future development.