Nionui

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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.

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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.