- Implemented FillLayerBackgroundCommand to fill the background of layers with a specified color.
- Introduced BackgroundFillManager to manage background fill operations.
- Updated LayerManager to include fillLayerBackground method.
- Enhanced LayersPanel with a color picker for filling layer backgrounds.
- Modified RasterizeLayerCommand to reflect changes in terminology and functionality.
- Adjusted LayerSort to ensure filled layers are rendered at the correct z-index.
- Updated relevant utility functions and components to support new fill feature.
