Bug 25402: Put OPAC cart download options into dropdown menu

Bug 25402: Put OPAC cart download options into dropdown menu

This patch modifies the process of downloading a cart so that it doesn't
involve a separate popup window.

To test, apply the patch and add some titles to the cart in the OPAC.

 - Open the cart
 - The "Download" button should now be a dropdown menu.
 - Selecting any of the download options should trigger a download
   prompt.
 - Test with and without some CSV profiles which have been configured
   for use in the OPAC

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>