Customization of HTML translation for object oriented definitions index

* tta/perl/Texinfo/Convert/Converter.pm (element_cdt),
tta/perl/Texinfo/Convert/HTML.pm (element_cdt),
tta/C/convert/convert_html.c (html_element_cdt_tree): add wrappers for
converters for translation based on element language information.

* tta/C/main/manipulate_indices.c (def_command_index_entry)
(index_content_element, index_entry_element_sort_string)
(index_entry_first_letter_text_or_command),
tta/perl/Texinfo/Indices.pm (_def_command_index_entry)
(index_content_element, index_entry_first_letter_text_or_command),
tta/perl/Texinfo/IndicesNonXS.pm (index_entry_element_sort_string):
pass converter in Perl down to def_command_index_entry and use
element_cdt if the converter is passed, and pass converter and a
function reference in C. Update callers.

* tta/C/main/manipulate_indices.c (index_entry_element_sort_string),
tta/perl/XSTexinfo/parser_document/IndicesXS.xs
(index_entry_element_sort_string): add debug level argument to
index_entry_element_sort_string.