Sort metadata list alphabetically by label regarding newspaper issues

[andre-hohmann]

Problem

When metadata is added for issues in the newspaper editor, the list of available metadata fields seems to be unsorted. Probably they are sorted by order in the ruleset or by internal labels. It is a similar behaviour as #3413, #3263 or #4656.
For users, it is difficult to find the correct label.

Solution

The metadata labels should be sorted in alphabetical order of the displayed labels.

Examples