desktop-entry: define an 'iconstring' type

Merged Will Thompson requested to merge wjt/xdg-specs:desktop-entry-iconstring into master

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 and the linked mailing list thread.

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