ResourceModel: made m_db pointer non-modifieable by subclasses

Subclasses have to call ResourceModel::db() now in order to get the
pointer to the DB, the model is operating on.

include/ResourceModel.h

@@ -73,6 +73,13 @@ public:
 
 
 protected:
+	ResourceDB * db() const
+	{
+		return m_db;
+	}
+
+
+private:
 	ResourceDB * m_db;