2024 08 23 layout refactor composer

[cmeyer]

• Eliminate root layout render trait class. Draw direct from layer render trait.
• Eliminate 'layout render trait' by merging into LayerCanvasItem class.
• Revert hack for repaint during layout. Will implement different solution.
• Remove unreachable sync_redraw code.
• Simplify updates by removing unused canvas items parameter and merging methods.
• Simplify layout and make it batchable.
• Change grid layout to use layout items passed into layout and get_size.
• Change scroll area to use content preferred size during first layout.
• Ensure update message goes up hierarchy whether current item has layout or not.
• Remove unused prepare_render methods.
• Add optional composer-style repainting to canvas item.
• Add canvas item composition composer.
• Rework checkbox canvas item to use composer.
• Change splitter to use composer.
• Change scroll area to use composer.
• Clean up drawing and layout to consistently use composer for both.
• Update slider, draw, background, divider, and progerss bar canvas items to use composer.
• Update list, grid canvas items to use composer.
• Change time stamp canvas item to use composer.
• Remove several unneeded layout methods.
• Remove several unneeded repaint methods.
• Add a canvas example app for demonstration and testing.
• Add ability to draw unique marker on canvas item for debugging.
• Fix issue with scroll area auto layout.
• Restore missing repaint_immediate function.
• Change update to only invalidate composer if canvas item visible.
• Update layer drawing to properly layout/update, including immediate mode.
• Add layer usage to canvas demo.
• Improve error handling and robustness when canvas item garbage collected while drawing.
• Improve layout to use layout copy so it does not get modified.
• Improve typing and add explanation for ordered layer thread repainting.
• Simplify 'needs repaint' logic.
• Add support for batch updates.
• Fix race condition when updating/invalidating composer from different threads.
• Consolidate button items subclassed from CellCanvasItem.
• Fix typo in canvas error logging.
• Add canvas item docs.