Nionui

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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.

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- Fix regression where line edits not using latest text when still focused.
- Improved look for basic (canvas item) tabs.
- Allow background color to be a gradient in most cases.
- Add ability to set specific borders and margins independently in declarative UI.

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- Add a bindable background_color property to widgets, requires tool 0.4.15 or later.
- Add a declarative twist-down section widget.
- Add Path object and method to add it to drawing context.
- Add a alternate compact Tab implementation.
- Add ability to bind text_color property of declarative label.
- Fix issue with dialogs being closed twice.