Fixes issue when actionPaneConfigurator is null when opening slidable

Fixes #512 and updates SDK and Flutter constraints

Cleanup and bump to 3.1.1

remove example/pubspec file

change sdk constraints

Merge pull request #417 from ueman/flutter-3.7

Compatibility with Flutter 3.7

update SDK constraints

Fixes #361

fixes #279

bump version 1.3.0