delete copy constructor and assignment operator of some internal classes

they are unused, but if anyone would use them things would go wrong, so protect us from it

src/imageformats/xcf.cpp

@@ -112,6 +112,9 @@ private:
         {
             delete[] name;
         }
+
+        Layer(const Layer &) = delete;
+        Layer &operator=(const Layer &) = delete;
     };
 
     /*!