Fixed missing links for topics without any name.
File upload: Add MD5 checksum.
Header cosmetics: [+] button needs no "Add..." text.
XTM import: Fixed fatal error on subjectLocatorRef.
Topic search page: Speed search results up a bit (with large number of topics, lookups become slow. We probably need a few database indexes.)
bin/xtm_import.php can now read filenames from STDIN.
XTM import: Ignore occurrences with an empty value.