Nionui

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- Minor changes for Python 3.12 compatibility.

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- Render SVG images with pixelated images.
- Many changes to layout engine to simplify and make it easier to use.

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- Fix issue with closing application, improve compatibility with Qt 6.5.
- Change fallback behavior for preferences to not use old preferences.
- Allow binding for placeholder text in line edit.
- Implement visible property to dock widgets.
- Eliminate spurious write to persistent storage when initializing section widget.
- Improve handling of wheel scrolling.

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- Add support for text browser (markdown), requires tool 0.4.18 or later.

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- Automatically activate windows when showing them.
- Use alternate low level asyncio on Windows to avoid crash/bug in default.
- Fix race condition in binding helper.

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- Introduce a Bitmap object to optionally replace uses of NDArray/uint32 bitmaps.