This code is extracted from goffice.

In particular,
- go-charmap-sel is a widget to allow the user to select an encoding
- go-option-menu is a widget used by go-charmap-sel to add manu items
  with a radio button.
- go-glib-extras is copied because we use a few functions from it:
  - go_ascii_strcase_equal and go_ascii_strcase_hash
  - go_guess_encoding

This code is currently used by the csv importer, but could be useful
in several interfaces that require the user to select an encoding
(various importers and exporters mostly).