gui: Use iconv/gnulib to save editor content in non-UTF-8. * bootstrap.conf: Add module "uniconv/u16-conv-to-enc". * liboctave/wrappers/uniconv-wrappers.c, liboctave/wrappers/uniconv-wrappers.h (octave_u16_conv_to_encoding, octave_u16_conv_to_encoding_strict): Add wrappers for gnulib function. * liboctave/util/oct-string.cc, liboctave/util/oct-string.h (octave::string::u16_to_encoding): Add new function. * libgui/src/m-editor/file-editor-tab.h (octave::file_editor_tab::check_valid_codec): Change return type of function. * libgui/src/m-editor/file-editor-tab.cc (octave::file_editor_tab::do_save_file): Use iconv/gnulib functions to save file editor content in non-UTF-8 encoding. The interpreter needs to be able to read these files using the iconv/gnulib functions anyway. And Qt6 can only convert to UTF-8, UTF-16, UTF-32 or the system locale encoding. (octave::file_editor_tab::check_valid_codec): Use iconv/gnulib facilities to check if selected encoding is valid for file editor content. (octave::file_editor_tab::save_file): Adjust for changed return type of check_valid_codec.

--- a/liboctave/wrappers/uniconv-wrappers.c	Fri Apr 14 21:24:06 2023 -0400
+++ b/liboctave/wrappers/uniconv-wrappers.c	Sat Apr 15 11:29:15 2023 +0200
@@ -93,6 +93,22 @@
 }
 
 char *
+octave_u16_conv_to_encoding (const char *tocode, const uint16_t *src,
+                             size_t srclen, size_t *lengthp)
+{
+  return u16_conv_to_encoding (tocode, iconveh_question_mark,
+                               src, srclen, NULL, NULL, lengthp);
+}
+
+char *
+octave_u16_conv_to_encoding_strict (const char *tocode, const uint16_t *src,
+                                    size_t srclen, size_t *lengthp)
+{
+  return u16_conv_to_encoding (tocode, iconveh_error,
+                               src, srclen, NULL, NULL, lengthp);
+}
+
+char *
 octave_u32_conv_to_encoding_strict (const char *tocode, const uint32_t *src,
                                     size_t srclen, size_t *lengthp)
 {