Modelio 3.x, 4.x

The UML validation ruleset should be extensible and manageable. The developer should be able to: * create a new rule (possibly inside a named ruleset) * modify an existing custom rule * activate/deactivate rules for validation

Rationale: For UML extensions (my usecase is the OntoUML Language), a custom set of validation rules needs to be defined which should be integrated into the Modelio processes. This allows creating custom model validation scenarios. Furthermore, it makes the contract between a model and the language behind Modelio explicit.