Slideshow updated with cycle2

Now it uses malsup cycle2 jQuery plugin (https://github.com/malsup/cycle2).

The size of the slideshow is now set as a ratio (3:2, 4:3, 16:9 ...) instead of pixels; this should help in responsive layouts.
Instead of the pager (which is still possible) arrows (prev.png and next.png) are shown overlayed.

For the many other options please see http://jquery.malsup.com/cycle2/

Changes:
* modified:   apps/filemanager/conf/embed.php
* new file:   apps/filemanager/css/cycle2.css
* modified:   apps/filemanager/handlers/slideshow.php
* new file:   apps/filemanager/js/jquery.cycle2.js
* new file:   apps/filemanager/js/jquery.cycle2.js.map
* modified:   apps/filemanager/views/slideshow.html
* new file:   files/design/next.png
* new file:   files/design/prev.png
* new file:   files/design/spinner.gif