Nionutils

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- Add ConcatStream and PropertyChangedEventStream.
- Add standardized [notify] item_content_changed event to Observable.
- Make item_changed_event optional for items within FilteredListModel.
- Add floordiv operator to IntSize.

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- Change list model text filter to use straight text rather than regular expressions.

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- Allow recorder object to be closed.
- Improve release of objects when closing MappedListModel.
- Add close method to ListModel for consistency.
- Allow persistent objects to delay writes and handle external data.
- Allow persistent relationships to define storage key.
- Extend Registry to allow registering same component with additional component types.

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- Allow default values in persistent factory callback.

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- Allow persistent items to be hidden (like properties).
- Allow persistent interface to use get_properties instead of properties attribute when saving.
- Allow FilteredListModel to have separate master/item property names.

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- Fix bug where unregistered objects were not reported correctly.
- Add model changed event to structured model to monitor deep changes.