SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

The last commit made m_partition unnecessary, and m_volume
has been unnecessary ever since the PR that added DiscExtractor.

This simplifies FileMonitor a lot and also lets us
clean up FilesystemPanel.

GC/Wii filesystem internals shouldn't be exposed to other classes.
This change isn't especially useful by itself, but it opens up the
way for some neat stuff in the following commits.

These prefixes were inconsistent with the rest of Dolphin.

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

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.

Advantages:

* Simpler code in general
* No extra volume objects created
* Now actually notices if the disc or partition gets
  changed while the core is running
* No longer picks up on disc access done by the GUI
  (it used to do so as long as the core was running)
* Gets rid of a Core dependency in DiscIO

There are two performance disadvantages:

* FileMonitor is now a bit slower when used with VolumeDirectory
  because FileMonitor now always uses the FileSystemGCWii code
  for finding filenames instead of VolumeDirectory finding the
  filename on its own and directly hooking into FileMonitor.
  But this isn't such a big deal, because it's happening on the
  DVD thread, and my currently unmerged file system PR will make
  FileSystemGCWii's file finding code about as fast as
  VolumeDirectory's.
* FileMonitor's creation of the file system object is now
  done on the CPU thread instead of the DVD thread, and
  it will be done even if FileMonitor logging is disabled.
  This will be fixed in the next commit.