Generation hack

okay, a next try
```C
typedef struct UA_VariableNode {
	UA_NodeId nodeId;
	UA_NodeClass nodeClass;
	UA_QualifiedName browseName;
	UA_LocalizedText displayName;
	UA_LocalizedText description;
	UA_UInt32 writeMask;
	UA_UInt32 userWriteMask;
	UA_Int32 referencesSize;
	UA_ReferenceNode* references;
	UA_Variant value;
	UA_NodeId dataType;
	UA_Int32 valueRank;
	UA_Int32 arrayDimensionsSize;
	UA_UInt32* arrayDimensions;
	UA_Byte accessLevel;
	UA_Byte userAccessLevel;
	UA_Double minimumSamplingInterval;
	UA_Boolean historizing;
	//I will not be seen on the wire, I will not be freed by _destroy
	UA_Open62541Data* Open62541Data;
} UA_VariableNode;
```

The definition of UA_Open62541Data is hidden  and will be used in ua_application.c like this
```
	//FIXME: not tested
	/*
	 struct UA_Open62541Data{
	 //FIXME: enum or union to come
	 	UA_Int32(*readCallback)(const UA_NodeId* nodeid, UA_Int32 typeId, const UA_Variant* value);
		UA_Int32(*writeCallback)(const UA_NodeId* nodeid, UA_Int32 typeId, const UA_Variant* value);
	 };
	 v->Open62541Data = (UA_Open62541Data*)malloc(sizeof(UA_Open62541Data));
	 free(v->Open62541Data);
	 */
```

We will probably need to source out the definition into a ua_opaque_types.c, since it will be used on different places: here and at least once 
* in the binaryEncode function, just before encoding the value, 
* or in the service just before reading the variable node

dunno yet. 

I'd probably go for calling callbacks form a service
* then we will not need to hack more generated code
* we will have the nodeId at hands at the service layer