Fix reading Wii FST size (for real this time)

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).

I forgot it when I added the method.

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.