Convert track names and input lines to NFC. This is a database format

change so a new read-only global preference _dbversion is introduced
to record the current database version.  Upgrade will implyy
renormalizing all keys and regenerating the search/tags databases.

Fiddled with tests to put both composed and decomposed non-ASCII names
through.

Document limitations on line syntax.  This really needs improving
before release since the same thing applies to the config file!

Make 'nothing' test quicker.