gui: Use iconv/gnulib to load editor content from files in any encoding. * bootstrap.conf: Add module "uniconv/u16-conv-from-enc". * liboctave/wrappers/uniconv-wrappers.c, liboctave/wrappers/uniconv-wrappers.h (octave_u16_conv_from_encoding, octave_u16_conv_from_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: Remove header that is no longer needed. * libgui/src/m-editor/file-editor-tab.cc (octave::file_editor_tab::load_file): Use iconv/gnulib functions to load file editor content from file in any encoding. The interpreter needs to be able to read these files using the iconv/gnulib functions anyway. And Qt6 can only convert from UTF-8, UTF-16, UTF-32 or the system locale encoding.

--- a/bootstrap.conf	Sat Apr 15 07:17:15 2023 -0700
+++ b/bootstrap.conf	Sat Apr 15 20:51:50 2023 +0200
@@ -105,6 +105,7 @@
   uname
   unicase/u8-tolower
   unicase/u8-toupper
+  uniconv/u16-conv-from-enc
   uniconv/u16-conv-to-enc
   uniconv/u32-conv-to-enc
   uniconv/u8-conv-from-enc