A save or export that writes a file per artboard, linked images or image
maps names them after the file picked in the save panel, which asked only
about that name. A file already there under one of the other names was
replaced without a question.
Save As, Save a Copy, Save as Template, Save and Export now check those
files before writing anything. When any is there, nothing is written and
a replaceFiles question names it (and lists all of them when there are
several). OK runs the save or export again on the document that asked and
replaces the named files; a file it would now write that the question did
not name is asked about again. Cancel writes nothing. A background save
or export checks from its worker and asks once it ends, after a dialog
open then. Files the document's last Save or Save As wrote are replaced
without asking.
Services::exists is now an Arc<dyn Fn(&str) -> bool + Send + Sync>.