decorator refactoring

- decorators' code move up to ./decorators
- one decorator should support each decoratable widget (or not :))
- PartBroswer and DiagramDesigner supports decorators so far
- DefaultDecorator supports both widget
- DecoratorWithPorts supports both widget
- CircleDecorator supports both widget
- AttributesDecorator supports only DiagramDesigner (in PartBrowser
DefeultDecorator replaces it)
- some other bugfixes related to the modification
decorator refactoring continued (should have been committed with the previous commit)