add xowiki::File content to the search index:
- extend xowiki method search_render to return "mime" and "text" fields
- xowiki::File->search-render calls now ::search::convert::binary_to_text
   to convert file content to index words (when available)
- use dict instead of associative array in ::xowiki::datasource
- fix typo (old bug, many thanks to Michael Aram for spotting this)