Reported by MrSynAckster. A specifically crafted NAND dump could use
path traversal to overwrite files on the host file system.

This is also an accuracy fix for importing NAND dumps that contain
file names that Dolphin is supposed to escape. Some games' save files
are affected.

GetPath has two special cases where it doesn't add a slash.

The first is for the root entry's special name "/". The next commit will
be neater if we can skip calling GetPath for the root entry, because '/'
is one of the characters that Common::EscapeFileName replaces with an
escape sequence. Let's check for entry number 0 instead.

The second is for parent paths that already end in a slash. There's no
actual need to check for this - double slashes are harmless, and for
comparison, NANDImporter::ExtractCertificates already appends slashes
without checking if there already is one. Let's remove this check.

Now with cancel button and an actual progress bar. For simplicity, we do
two passes on the progress bar, one for loading the NAND into memory and
one for extracting it. The user directory is likely on an SSD, making
the extraction pass invisibly fast.

The new return value is `std::ranges::subrange`.