This saves us from having to update the GameFileCache when the
TitleDatabase changes (for instance when the user changes language).

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.

The advantage of std::list is that elements can be removed from the
middle efficiently, but we don't actually need that, because the
ordering of the elements doesn't matter for us. We can just replace the
element we want to remove with the last element and then call pop_back.

Replacing list with vector should speed up looping through the elements.

Reduces the number of files that have to be recompiled
when changing the cache revision.

This was a regression in f49b64c. Some games seem to name the
banner file OPENING.BNR instead of opening.bnr.

Should fix https://bugs.dolphin-emu.org/issues/10354