* Searching '-mytag' will now exlude all shaares with 'mytag' tag.
* All tags starting with a '-' are renamed without it (through the Updater).
* Unit tests.
Minor code changes:
* LinkDB->filter() can now take no parameters (get all link depending on logged status).
* tagsStrToArray() is now static and filters blank tags.
* 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.
