ISOFile and GameFile were using IsWiiDisc() and IsWadFile() to set
an enum value. The volume might as well return an enum directly.

I increased the Qt CACHE_REVISION because m_platform now is saved as u32
instead of int, but increasing the wx CACHE_REVISION is not necessary.

Disc number is changed from bool to u8, and revision is changed from
int to u16 (WADs can use all 16 bits, but discs can only use 8 bits).

Replaces them with forward declarations of used types, or removes them entirely if they aren't used at all. This also replaces certain Common headers with less inclusive ones (in terms of definitions they pull in).

Having some data available in banner loaders and some other data
data available in volumes gets messy, especially with GetNames(),
which is available in both but returns different results
depending on which one is used. This change drops support
for reading names and descriptions from Wii save data.

This makes the code cleaner and also leads to some user-visible changes:

The wx game properties will no longer let the user
select WAD languages that don't have any names.

The Qt game list will now display names using the languages
set in the configuration instead of always using
English for PAL GC games and Japanese for WADs.

If a WAD doesn't have a name in the user's preferred language,
English is now selected as a fallback before Japanese.

This is good hygiene, and also happens to be required to build Dolphin
using Clang modules.

(Under this setup, each header file becomes a module, and each #include
is automatically translated to a module import.  Recursive includes
still leak through (by default), but modules are compiled independently,
and can't depend on defines or types having previously been set up.  The
main reason to retrofit it onto Dolphin is compilation performance - no
more textual includes whatsoever, rather than putting a few blessed
common headers into a PCH.  Unfortunately, I found multiple Clang bugs
while trying to build Dolphin this way, so it's not ready yet, but I can
start with this prerequisite.)