eurokey layout for xkb implementing version 1.3

This eurokey layout for linux xkb implements all dead keys and mappings of eurokey version 1.3.

layout changes

• large ẞ on altgr+shift+s
• altgr-minus and altgr-underline map to ✓ and ✗
• dead-greek layer on altgr+m

additional features with custom xcompose file

• dead notsign implementing arrow keys, percentages and trademark signs
• dead mathematical shift-altgr on altgr+shift+m

This is completely based on the eurkey layout by Steffen Brüntjen under GPLv3.