texi2html: Use the element_target_name function to set the correct anchors

texi2html: Use the element_target_name function to set the correct anchors

-) rename split_at_numbered_section to lilypond_element_file_name
-) texi2html now added the element_target_name function, which is the proper
   interface to modify the anchors from SECxx to our way (anchor derived from
   node name or section name if no node is attached).
-) Removed the 'target' and 'id' modification from element_file_name, since
   it's now done in element_target_name
-) Use the 'target' for anchors in the TOC instead of the 'id'

There is still an issue left in texi2html: For sections, it prints out the
'id' as the anchor instead of the 'target', so I also need to modify the 'id'.
Once texi2html is fixed, remove that from the element_target_name function.