Optimize default wallpaper

As mentioned in artwork/public#18 (comment 97912)

An attempt to minify the default wallpaper file, without any rendering differences.

Motivation - since this wallpaper is voted to become the default wallpaper, I believe it would be good to have it as small as it can be, to lighten up any resource usage for the first-time use experience. Even for the smallest files like this - the original file (with metadata added) is 9298 bytes, optimized - 6531 bytes (29.76% reduction).

The following Imagemagick command produces a fully transparent image, meaning there are no rendered pixel difference.

compare backgrounds/xfce-x.svg backgrounds/xfce-x-original.svg -compose src diff.png

A huge part in minification is the removal of Inkscape generated IDs, namespaces, and other attributes. Inkscape license allows doing any alteration to images produced, without attribution.

There were also some duplicated gradient definitions removal. Unclear robotic-generated ID names were either removed, or renamed to have sense.