This lets VolumeDirectory/DirectoryBlob skip implementing
various volume functions like GetGameID, GetBanner, etc.
It also lets us view extracted discs in the game list.

This ends up breaking the boot process for Wii
DirectoryBlobs due to workarounds being removed from the
boot process, but that will be fixed later by adding
proper DirectoryBlob support for things like TMDs.

We now expect the directories to be laid out in a certain
format (based on the format that WIT uses) instead of requiring
the user to set the DVD root and apploader path settings.

Reduces the number of files that need to be recompiled
when making changes to FileUtil.h.

These prefixes were inconsistent with the rest of Dolphin.

I'm also renaming VolumeWiiCrypted to VolumeWii because of 1113b13.

Normal users don't care about it. In fact, people care so
little about it that the Wii implementation of it was broken
starting from when it was implemented (eb65601) to 7 years
later (e0a47c1), apparently without anyone reporting it.

IIRC we were only using {{}} because {} used to cause errors
on the FreeBSD buildbot.

By removing mutable state in VolumeWiiCrypted, this change makes
partition-related code simpler. It also gets rid of other ugly things,
like ISOProperties's "over 9000" loop that creates a list of
partitions by trying possible combinations, and DiscScrubber's
volume swapping that recreates the entire volume when it needs to
change partition.