#3218 / download.php / export

Implemented download.php to offer SIP protected downloads for single files (any filetype) as well as concatenated PDF files and converted HTML pages.
download.php: API Interface
DownloadException.php: New exception class for downloads - might be extended for better error handling.
OnArray.php: new function getArrayItemKeyNameStartWith() to filter for specific elements in an array. New function arrayEscapeshellarg() to escape args
Download.php: Main class.
Link.php, Report.php: implemented new link type 'd' (=download)