Don't read from volume when reloading Wii banners

Should make loading cached Wii games that lack banners slightly faster.

In the past, only GCZ was shown as compressed, not CISO and WBFS.

04fcb72 fixed an issue with reading the Wii FST size, but I found a second
issue when working on PR #2820 - the size must be shifted left by 2.
DiscScrubber and Boot already do this correctly using separate code.

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).