* tta/C/main/manipulate_indices.c: rename cdt_element_tree_fn as
element_cdt_tree_fn.

* tta/C/convert/converter.c (converter_format_data)
(get_converter_indices_sorted_by_index)
(get_converter_indices_sorted_by_letter), tta/C/convert/converter.h
(CONVERTER_FORMAT_DATA): add element_cdt_tree field in
CONVERTER_FORMAT_DATA, set it for HTML and get it in
get_converter_indices_sorted_by_*.

* tta/C/convert/texinfo.c (txi_complete_document),
tta/C/main/document.c (document_indices_sort_strings)
(sorted_indices_by_index, sorted_indices_by_letter)
(print_document_indices_sort_strings),
tta/C/main/manipulate_indices.c (index_entry_element_sort_string)
(setup_index_entries_sort_strings)
(setup_sort_sortable_strings_collator, sort_indices_by_index)
(sort_indices_by_letter), tta/perl/Texinfo/DocumentNonXS.pm
(print_document_indices_sort_strings),
tta/perl/Texinfo/IndicesNonXS.pm (index_entry_element_sort_string),
tta/perl/XSTexinfo/parser_document/DocumentXS.xs
(setup_indices_sort_strings, indices_sort_strings)
(print_document_indices_sort_strings), tta/swig/texinfo.i:
pass converter in Perl and converter and HTML specific function from
index sorting and print_document_indices_sort_strings down to
indices_sort_strings or index_content_element.  Update callers.

* tta/C/convert/html_prepare_converter.c (html_converter_customize)
(html_conversion_initialization): set self->current_* in
html_converter_customize and not right before starting to output, as
the current_* struct fields may be used in code called before starting
the conversion, mainly for index strings sorting to use a customized
function for translation.

* tta/C/texi2any.c (main), tta/perl/t/test_utils.pl (test),
tta/perl/texi2any.pl: setup converter earlier to have it for the call
to setup_indices_sort_strings or txi_complete_document.

* tta/perl/Texinfo/Document.pm (BEGIN), tta/perl/XSTexinfo/Makefile.am
(DocumentXS_la_CPPFLAGS, DocumentXS_la_LIBADD): link DocumentXS
against conversion library now that a converter and associated
functions may be needed for index strings sorting translation
customization.