Event participant (and possibly other entity) imports do not handle "non-comma CSV"

As reported by Aivars and Allison in

• SE: Can't import Event participants
• SE: Importing Participants
• SE: Import field separator bug

Import Event Participants (and apparently some other import functionality) suggests in the UI that it accepts delimited files with non-comma delimiters, but does not appear to function when supplied such data.

I tested this by -

• Visit demo.civicrm.org Drupal
• Search for event participants
• Export the participants for Rainforest Cup
• Save as CSV, replace commas with semicolons
• Import as "CSV" of event participants using semicolon as delimiter
• All rows are reported as invalid, as though no matching of columns to fields was supplied

Looks like CiviCRM ignores the stated file upload delimiter? I tested Contact import and it worked OK with semicolon-delimited file for a trivial test, but according to the SE question this is an issue on Event Participant and Contribution importers, so quite possibly others also.

Also - while we're at it - all the importers seem to contain help text (below the file upload dialog) suggesting that the file should be comma separated, AND an input field to provide the file's delimiter at the bottom of the field. Only one of these should be the case?

Personally I don't mind if we just tell people to use commas, it's not hard, depends if anyone wants to make the effort to support *SV ... maybe if someone needs to import lots of files that come in as TSV they will want to sponsor this work