Allowed block widgets to be dragged and dropped in nested editables.
Check nested editable filter to make sure widget can be dropped there.
Don't let the widget to be dropped into its own nested editable.
Tests: Extracted lineutils tools with lineutilsTestsTools#assertRelations method.
Tests: Added test for dropping widgets into nested editable of restricted filter.
Tests: Added tests for widget drop into nested editables: of no specific filter and own nested editable. Minor test refactoring.
Changelog entry.
Merge branch 't/12006' into major