Add gradient support to font formatting

- Introduced gradient properties in FontFormat: enabled state, start color, end color, angle, and size.
- Implemented gradient handling functions in fontformat_commands.py.
- Updated funcmaps.py to include new gradient handlers.
- Enhanced FontFormatPanel in text_panel.py with UI controls for gradient settings (enable checkbox, color pickers, angle slider, size slider).
- Modified TextBlkItem in textitem.py to support gradient rendering in the paint method.
- Ensured gradient properties are preserved during font format updates.

This commit enhances the text formatting capabilities by allowing users to apply gradients to text elements, improving visual customization options.