[Xfce-i18n] Xfce4-about reworked

[Nick Schermer]

Ok, faster then I thought, but since we're going to pack pre1 tomorrow
and make some eleases for build fixes (xfce4-settings, xfce-utils and
garcon), I've merged the branch so the changes can go into pre1. Some
new strings to translate in xfce-utils, no need to hurry tho, enough
time to translate after pre1.

https://translations.xfce.org/projects/p/xfce-utils/c/master/

Nick