Overhaul of the indexing module

1. Make it possible to search for index codes by their content and enable editing of such codes.(instead of having to recreate them from scratch when spotting a mistake).2. Allow the use of attributes such as italics in the index codes; this is essential when indexing titles of works.3. Increase the number of indexing levels beyond two, and give different numerical values to the codes for each level to enable a macro to determine the level of an index code to the left or right of the cursor.4. Support multiple indexes.5. Support indexing according to automatic paragraph numbers rather than only pages.6. Support cross-references ("see" references").7. Make it possible to index according to automatic paragraph numbers rather than only to pages.8. Possibility of indexing the contents of footnotes, which would return entries like 252n4 (or 252 n. 4).