These disc images are only used on dev units and not retail units.
There are two important differences compared to normal Wii disc images:

- The data starts 0x8000 bytes into each partition instead of 0x20000
- The data of a partition is stored unencrypted and contains no hashes

Our old implementation was just guesswork and doesn't work at all.
According to testing by GerbilSoft, this commit's implementation
is able to read and extract files in the filesystem correctly,
but the tested game still isn't able to boot. (It's thanks to their
info about unencrypted disc images that I was able to make this commit.)

Makes the enum strongly typed. A function for retrieving the string
representation of the enum is also added, which allows hiding the array
that contains all of the strings from view (i.e. we operate on the API,
not the exposed internals). This also allows us to bounds check any
querying for the strings.

This isn't really useful for anything anymore as far as I know.

Also removing some prefixes that we don't need now that we're
using enum classes instead of plain enums.

Reduces the number of strings to translate.

The original reason I wanted to do this was so that we can replace
the Android-specific code with this in the future, but of course,
just deduplicating between DolphinWX and DolphinQt2 is nice too.

Fixes:

- DolphinQt2 showing the wrong size for split WBFS disc images.

- DolphinQt2 being case sensitive when checking if a file is a DOL/ELF.

- DolphinQt2 not detecting when a Wii banner has become available
after the game list cache was created.

Removes:

- DolphinWX's ability to load PNGs as custom banners. But it was
already rather broken (see https://bugs.dolphin-emu.org/issues/10365
and https://bugs.dolphin-emu.org/issues/10366). The reason I removed
this was because PNG decoding relied on wx code and we don't have any
good non-wx/Qt code for loading PNG files right now (let's not use
SOIL), but we should be able to use libpng directly to implement PNG
loading in the future.

- DolphinQt2's ability to ignore a cached game if the last modified
time differs. We currently don't have a non-wx/Qt way to get the time.

Add a way to easily get the title ID of a game. For Wii titles, it's
sometimes more useful to know than the game ID.