Introduce ImageList.copy()

- imagelist[:] returns a list, not an ImageList
- it returns an instance of ImageList

picard/metadata.py

@@ -275,7 +275,7 @@ class Metadata(MutableMapping):
                 del self[tag]
 
             if other.images:
-                self.images = other.images[:]
+                self.images = other.images.copy()
             if other.length:
                 self.length = other.length
 

picard/util/imagelist.py

@@ -51,6 +51,9 @@ class ImageList(MutableSequence):
     def __eq__(self, other):
         return self._sorted() == other._sorted()
 
+    def copy(self):
+        return self.__class__(self._images)
+
     def get_front_image(self):
         for img in self:
             if img.is_front_image():