OWAssociate: refactor

Associate widget needs refactoring.

- [ ] split the widget part from the model and the selection (see Orange widgets for reference)
- [x] check if scatter plot is used anywhere, if not, remove references
- [ ] organize code in logical order (set_data, then commit, then send_report)
- [ ] write tests

Start by locating the O WAssociate widget, its model, selection code, and any scatter-plot references; compare the structure with the Orange widgets mentioned in the issue. Review the set_data, commit, and send_report flow, then identify the existing test entry points. Done means the widget, model, and selection are separated, code is logically ordered, unused scatter-plot references are removed, and tests cover the refactored behavior.

Written by the indexing model from the issue text.