Short titles fit better than long titles.

GC games with long names store two variations of the name in
opening.bnr. This makes the shorter of those names available.
For volumes other than GC discs, prefer_long is ignored.

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.

Volume: Use more appropriate types for some returned values

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).

Eventually, netplay will be able to use the host's NAND, but this could
still be useful in some cases; for TAS it definitely makes sense to have
a way to avoid using any preexisting NAND.

In terms of implementation: remove D_WIIUSER_IDX, which was just WIIROOT
+ "/", as well as some other indices which are pointless to have as
separate variables rather than just using the actual path (fixed, since
they're actual Wii NAND paths) at the call site.  Then split off
D_SESSION_WIIROOT_IDX, which can point to the dummy NAND directory, from
D_WIIROOT_IDX, which always points to the "real" one the user
configured.

Use an ID-to-name map when volume has no company string

This is written so that the result of GetCompanyFromID never is cached
(except on Android?). Caching is unnecessary because the string can be
obtained quickly at runtime, and not caching it means that the cache
doesn't have to be invalidated when GetCompanyFromID is edited.

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).