The in-app browser that chooses a library folder on the server could
only open folders that already existed, so a library, or an export
destination, that was not on the server yet had to be made in the
Nextcloud web page first. It now has "New folder": a name, then MKCOL,
then the parent listed again and the new folder walked into — a folder
somebody has just named is the one they mean to choose.
The listing is the server's rather than the name inserted locally: the
server may have normalised or refused it. A name with a slash, "..",
or nothing at all is refused before any request, because a folder
typed with a slash in it is a path the user did not mean.
remote_folders holds the two WebDAV round trips (list, make) off the UI
thread, with the answer delivered through a Slint timer, so the album
sheet can use the same browser.