New Xfce 4.20 Wallpapers

@farhangbakhshi, @rowie324, @denys and @kazz:

I added metadata elements (Author and License) to your pictures.

Please check if it is fine for you like that / let me know in case you want to e.g. use a different dc:creator name.

I as well renamed your pictures, in order to fit into the existing name-scheme.

See as well artwork/public#18 (closed)