Nionui

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- Minor code maintenance.

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- Introduce Declarative.Handler base class. Optional, but will be required in 0.7.0 and later.

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- Allow setting of widget id in declarative; useful for testing.
- Allow binding of image property in declarative image (33).
- Ensure handlers are closed properly (50).
- Fix namespace issue.

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- Fix issue with spacing items in column/row widgets.

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- Add ability to set color/font/word_wrap on declarative UI labels.
- Enable strict typing on entire library.
- Drop support for Python 3.7, enable support for Python 3.10.
- Add methods to create persistent value models.
- Allow subclasses to override persistent value behavior.
- Configure stack widgets to expand/contract as content changes.
- Fix issue with declarative label width.
- Extend declarative components to make it easier to implement all-in-one handler+view.
- Improve handling of focus in dock widgets.
- Add divider and draw canvas items and declarative divider.
- Standardize to use a single event loop managed by Application.
- Add canvas item slider.

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- Fix radio button grouping issue.
- Improve font scaling behavior on Windows when running PyQt or PySide.
- Add a window.activate method.
- Expand action capabilities and be able to match key sequences.
- Add additional border color support for declarative images.
- Add icon based declarative radio buttons.
- Fix issue with lists making the selection visible.
- Remove menus from utility windows (affects Windows and Linux).
- Fix case of preference dialog when no preference panels are registered.