- Remove all Dublin Core-specific mirroring and out-of-form merge logic;
  the plugin now works on stock Piwigo with no other plugins installed.
- Add a public 'autosavemeta:saved' DOM event dispatched on the form
  after each background save, so other plugins (including Dublin Core)
  can react and refresh their own on-screen fields.
- Fix plugin description: it submits the form via fetch, not
  pwg.images.setInfo.