* contains methods designed to be run once.
* is able to upgrade the datastore or the configuration.
* is based on methods names, stored in a text file with ';' separator (updates.txt).
* begins with existing function 'mergeDeprecatedConfigFile()' (options.php).
* introduce class LinkFilter to handle link filter operation (and lighten LinkDB).
* handle 'private only' in filtering.
* update template to prefill search fields with current search terms.
* coding style.
* unit test (mostly move from LinkDB to LinkFilter).
PS: preparation for #358#315 and 'AND' search.