German online help for 8.12.0, round#1

OXUD-1164 Story OXUIB-2203 Planning view opened from appointment create should not offer "save as distribution list"
OXUD-1168 Update the docu about how appointments are displayed
OXUD-1175 Some minor fixes and improvements
OXUD-1180 Story OXUI-1172 As a user I can configure when unread emails get marked as read
OXUD-1177 Story GUARD-389 Implement S/Mime for 8.0
OXUD-1183 Enhancement: Update the docu of searching by PIM Categories