Icons are currently of type 'localestring', which is defined by this specification as follows:
Values of type 'localestring' are user displayable, and are encoded in UTF-8
Icon names are not user-displayable, and so should not be extracted by tools like xgettext for translation along with user-displayable strings. However, we still want tools that load .desktop files to respect the (rare) cases where icons have been localized. Define a new type, 'iconstring', with the properties we want.
See discussion at https://savannah.gnu.org/bugs/index.php?56543 and the linked mailing list thread.