Support generalizations and dependencies between datatypes:

umbrello/assocrules.cpp
- In function allowAssociation(assocType, widget) return false if
  widgetType is wt_Datatype and Model_Utils::isCommonDataType() returns
  true for widget->name(). Reason: It shall not be possible to add an
  association originating from a programming language predefined type.
- In m_AssocRules[] add entry for
  - Generalization with source wt_Datatype and target wt_Datatype;
  - Dependency with source wt_Datatype and target wt_Datatype.