WIP Update cms-eventdisplay-files.xml

Addressing issues: https://github.com/cernopendata/opendata.cern.ch/issues/311  https://github.com/cernopendata/opendata.cern.ch/issues/154 https://github.com/cernopendata/opendata.cern.ch/issues/333 and https://github.com/cernopendata/opendata.cern.ch/issues/223
OK, then I'll wait with the merge until everything is ready.  I've put WIP in the PR title; please remove when ready.

BTW it would be better to make separate PRs not in a file-by-file manner, but in change-by-change manner:

- one PR to amend the meaning of 256 everywhere (touching many files)
- one PR to amend 774/581 in event display files (touching this file only)
- ...