Nionui

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- Avoid potential race of requesting update while doing layout.
- Fix 95. Update selection when items inserted removed from list.

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- Add list canvas item 2, using canvas items for optimized rendering.
- Add visible_rect to repaint methods to help optimize repainting.
- Add an unrestrained sizing option to relax restraint from contained canvas items.
- Support sizing properties on minimal-style push button.
- Fix race condition resizing root canvas items.
- Change list widget and stack widget to use a None index as 'unselected' instead of -1.
- Extend background canvas item to allow drawing no background via a fallback color.
- Clean up scroll area auto sizing.

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- Rework canvas item layout/paint architecture for improved performance.
- Add a nionui module for easier launching.
- Add support for text alignment in labels.
- Add create_persistent_bool_model for persistent bool storage.
- Add support for timestamp_ns for display timing measurements.
- Change ComboBox to stop scrolling when not expanded.
- Use modifiers at mouse click rather than mouse release for modifying focus.
- Update for numpy 2.
- Add Python 3.13 support. Drop Python 3.9, 3.10 support.
- Add canvas item demo code.

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- Add support for pyside6 (alternate Qt 6.7 frontend) as a successor to pyside2.
- Refactor canvas item layout code to be non-threaded. Deprecate prepare_render method.
- Fix several race conditions in canvas drawing code.
- Improve error reporting in UI callbacks.

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- Minor addition for future color scheme change support.

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- Add support for handling screen scaling changes.
- Add support for declarative text browser (with markdown).
- Add support for grid item tool tips.
- Improve handling of tool tips (hide when mouse leaves canvas item).