keyboard/altgr: eurkey version 0α from Xmodmap

Copied from official (outdated) Linux download at:
https://eurkey.steffen.bruentjen.eu/download/xmodmap/0/Xmodmap_alpha

Cover image of fan-made hardware from <https://blog.psy-q.ch/blog/2014/11/25
/presenting-the-eurkeyboard-a-mechanical-keyboard-for-europeans-and-coders/>