Right now, apps have to implement their own stuff to check whether a file exists
and whether the user wants to overwrite it.

We could default to presenting such confirmation in SAVE/CREATE_FOLDER mode, and
have a way to disable it for apps that need exotic stuff.  The default would
ideally be to present automatic confirmation, although existing apps would have
to turn it *off* so that their dialogs keep working, or modify their code not to
do their own confirmation.