Header editor (GUI) and mandatory elements with default values
this is a followup to #2241 (closed): If one has a mandatory element with a default value, the GUI contains the text "This element is currently present in the file. It cannot be removed because it is a mandatory header field." The second sentence isn't true if the element has a default value. And since #2241 (closed) it is also inconsistent because in the very next line MKVToolNix offers to remove said element.