Replaced NBSP constant with a new nbsp() function. The return value of HTML::nbsp...

Replaced NBSP constant with a new nbsp() function. The return value of HTML::nbsp() depends on the CHARSET in index.php. It provides the utf-8 and iso-8859-1 non-breaking spaces, and for any other charset it falls back to the html-entity " ". (Maybe this should just always output an html entity? We currently use a raw "\xA0" character for iso-8859-1 so this value was left as is.)

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@2401 96ab9672-09ca-45d6-a79d-3d69d39ca109