Added a reference to the media-manager where you can see on which pages the file is used.

This change contains the same code as in #321 but for the media-manager. 

It was proposed in https://forum.dokuwiki.org/thread/13784

Implements
https://bugs.dokuwiki.org/index.php?do=details&task_id=2525

In an older version of PHP a file_exists() call would issue a warning
when the file did not exist. This was fixed in later PHP releases. Since
we require PHP 5.3 now, there's no need to supress any error here
anymore. This might even give a minor performance boost.

many PHPDocs
some unused variables
some dynamically declared variables declared

different types
unused vars
PHPDocs

as discussed in 22db8df7

Sometime the passed AUTH was not correct. This is a quick fix,
but the whole media manager handling is one stining pile of crap that
needs a major rewrite.

fix fs#2851
related to http://bugs.jqueryui.com/ticket/8761

this file makes me cry. there is so much shit going on here.

http://english.stackexchange.com/questions/117552/why-does-information-not-have-a-plural-form

do not capture groups in media_isexternal. saves perhaps some cpu cycles and some bytes memory

also allow the order to be specified in more places.

Currently the search box is below the media file list, where it is
effectively inaccessible in a namespace with lots of files.  This
change moves the search box above the media file list and below the
upload form.