gui: Show encodings available with iconv in file editor preferences. * liboctave/wrappers/iconv-wrappers.h, liboctave/wrappers/iconv-wrappers.c (octave_iconvlist_wrapper, octave_iconv_canonicalize_wrapper): Add wrappers for libiconv functions. * liboctave/util/oct-string.h, liboctave/util/oct-string.cc (octave::string::get_encoding_list): Add new function that returns an ordered list of canonicalized encoding names that are available from libiconv. Use list of encoding identifiers as fallback on platforms without the required funcions. * libgui/src/gui-settings.cc, libgui/src/gui-settings.h (octave::gui_settings::get_codecs): Remove function. (octave::gui_settings::combo_encoding): Show list of encoding names that are actually available to the interpreter instead of a list of encodings available in Qt. * libgui/src/qt-interpreter-events.cc (octave::qt_interpreter_events::gui_preference_adjust): Remove logic for mapping between Qt encoding names and iconv encoding names. * libgui/src/settings-dialog.cc: Remove unused header. * m4/acinclude.m4 (OCTAVE_CHECK_ICONVLIST, OCTAVE_CHECK_ICONV_CANONICALIZE): Add checks for functions from libiconv that are not available on all platforms. * configure.ac: Call new functions.

root = true

[*]
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[*.{am,mk}]
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