improve qsearch suggestions

* support namespaces with more then one level e.g. @your:namespace
* support ns:<ns> as well

Currently, we don't support namespaces like: `kaugtöö:` or `星期一test:` in the qsearch.
While the `ft_pageSearch()` via `ft_queryParser()` breaks the search query and matches with `/^(?:\^|-ns:)(.+)$/u` the separated terms. That handles these example namespaces fine.
https://github.com/splitbrain/dokuwiki/blob/master/inc/fulltext.php#L595

Using something like `'/(?:^| )(?:@|ns:)(.+?\b)/'` in `_ft_pageLookup()` breaks also at the non-latin characters. Has someone a better regexp?