Fix exception introduced in d765211

```
  File "picard/ui/metadatabox/edittagdialog.py", line 222, in _get_supported_tags
    tags = sorted(list(tag_names()) + self.metadata_box.tag_diff.tag_names)
                  ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'set' and 'list'
```

This issue was introduced in commit d765211, it should have been `list()` instead of `set()`, tag names are already unique, but `tag_names()` is a generator.

Author: zas

picard/ui/metadatabox/edittagdialog.py

@@ -219,7 +219,7 @@ def _get_supported_tags(self):
         Returns:
             List of supported tag names
         """
-        tags = sorted(set(tag_names()) + self.metadata_box.tag_diff.tag_names)
+        tags = sorted(list(tag_names()) + self.metadata_box.tag_diff.tag_names)
         if len(self.metadata_box.files) == 1:
             current_file = list(self.metadata_box.files)[0]
             tags = list(filter(current_file.supports_tag, tags))