ModelEditor: Introduce custom relation type "Connection"

Change-Id: Iddd45508dcb2de1fa2b6d9b2b881b54a73172caf
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/modelinglib/qmt/diagram_controller/dupdatevisitor.h

@@ -52,6 +52,7 @@ public:
     void visitMDependency(const MDependency *dependency) override;
     void visitMInheritance(const MInheritance *inheritance) override;
     void visitMAssociation(const MAssociation *association) override;
+    void visitMConnection(const MConnection *connection) override;
 
 private:
     bool isUpdating(bool valueChanged);