CHG: Moved some functionality of the download box into functions.

* This improves readability, and allows to reuse the code.


git-svn-id: http://svn.montala.net/svn/resourcespace@4979 c08608d7-6e46-0410-86ca-f2a6f1370df5
ADD: format_chooser plugin

* Allows to change the format for a download by replacing the normal download
  size options with drop-down menus for size and format.
* This is similar to transform, but can be used in addition to it as well.
* If the file already exists in the specified format and size, it will be
  downloaded directly, if not, it will be converted on the fly (as a temporary
  file that will be removed after having downloaded it). A cache could be
  implemented, too, though.
* Support for collection_download.php is not yet implemented.


git-svn-id: http://svn.montala.net/svn/resourcespace@4980 c08608d7-6e46-0410-86ca-f2a6f1370df5