Nionui

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- Improve capabilities of window closing workflow.
- Fix issues with file dialogs in PyQt version.

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- Add support for declarative pop-up window (preliminary).
- Improve key handling in list widget.
- Improve handling of slider value to avoid update cycles in multiple sliders bound to same value.
- Spacing in item-bound rows/columns now works.
- Add a declarative image widget.
- Allow custom bindings in declarative handlers.
- Allow binding to push button and check box text content.
- Add support for declarative polymorphic components using get_resource and examples.
- Add support for declarative list box.
- Add support for specifying expanding declarative items (horizontal/vertical-size-policy).
- Add show_ok_dialog and show_ok_cancel_dialog methods to application class.
- Add run_ui to application for running simple declarative windows.
- Add functions for opening file dialogs without a window.
- Add support for adjusting menus at the application level.
- Add function to truncate string to pixel width.
- Add color/font methods to eliminate need for stylesheet properties.
- Improve handling of scrolling when changing list selection.
- Add UI function to retrieve default cursor tolerance for hit testing.
- Add basic notification dialog when actions report warnings/errors.
- Consolidate default behavior (dock windows, window closing, etc.).
- Add backend support for multi-threaded section-serialized rendering.
- Add backend support for section by section drawing for improved performance.
- Add latency display capability with rolling average to backend.
- Fix drawing bugs (nested layers, startup race condition, thread shutdown).
- Introduce action architecture for declarative menus and key bindings.
- Add support for "name" keyword for declarative row and column widgets.

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- Fix tool tip handling (incorrect recursive implementation caused UI hangs).
- Add support for layer caching (optimized drawing, part 1).

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- Add support for dynamic tool tips in canvas items and list items.
- Change default logging level to INFO (was DEBUG).
- Add ability to register declarative components.
- Add preliminary support for PySide2 (similar to PyQt).
- Add unbind methods to complement all bind methods.

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- Add icon to push button. Add binding to both push button text and icon.
- Add binding to enabled/visible/tool_tip and size properties for all declarative elements.
- Fix minor checkbox issue in PyQt.
- Improve ability to handle stacked canvas items during drag and drop.
- Extend list canvas item to support drag and drop on items.
- Fix issues with SVG 1.1 compatibility (use 'none' in place of 'transparent', opacity).
- Do not select list item if click handled in delegate mouse_pressed_in_item method.

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- Fix problem clearing tasks. Add ability to clear queued tasks too.
- Fix problem leaking threads in PyCanvas in PyQt backend.
- Extend sizing policy support.
- Expand capabilities of StringListWidget. Stricter keyword arguments too.
- Implement high quality image rendering in PyQt backend.