* Fix selector in subtree loading callback
  * Remove HTML inconsistencies between AJAX and plain PHP lists
  * Unify icon and CSS class switching in dw_tree and dw_mediamanager

don't use form_makeTag() for blank.gif as empty attributes are not passed on (which resulted in a missing alt attribute)

Attention: $lang['register'] has been renamed to $lang['btn_register'],
anyone using that in any plugin or template should adjust it.

This patch allows to use the placeholders in the search intro message that
will be replaced with the search term.

@SEARCH@ will be replaced with the search query
@QUERY@  will be replaced with the URL encoded search query for use in URL
         parameters

Please note that the replacement is don't on the XHTML *after* parsing and
rendering the intro wiki text. This means you can not use the query where
an ID would be expected.

Examples:

  This will work:
    [[http://www.google.com/search?q=@QUERY@|Google for @SEARCH@]].

  This will not work and will link to the page "search" instead:
    [[@SEARCH@|Your page]].

  You could use this instead:
    [[this>doku.php?id=@QUERY@|Your page]].

This reverts commit fa7c70ff.

This commit fixes two bugs that occurred when msg() was called after
html_msgarea() had already been called.
- the $MSG array is now cleared when it has been printed (otherwise $MSG
  has been printed again when another msg() call was done)
- headers_sent() didn't work for me, it always reported false although
  html_msgarea() had already been called which might be explainable with
  output buffering. This makes msg() now depend on the first call of
  html_msgarea() or headers_sent() in order to not to break msg() in
  ajax requests etc.