Background handling improvements

[Thomas Wood <thos gnome org>]

I've just finished improving the support for background categories by 
centralising the list of categories for all pages to use. The list of 
categories is now controlled in all places from the 
$background_config_array in includes/config.inc.php.

I've also added support for SVG images, based on the patch from Henrik.

There has been interest expressed for a larger preview on backgrounds, 
possibly scaled at around 640 pixels width. If possible, I'd like to 
actually create a pop-up preview window, so that previews don't link 
straight through to an image.

Any other comments on this, or any other background handling issues?


-Thomas