All we're really doing is iterating over a sequence of strings, so we
don't need to tie this specifically to std::vector.

std::function is internally allowed to allocate, and these functions
aren't being stored anywhere (only called), so we can freely get rid
of some minor overhead here by passing by reference.

This change also creates aliases for the functions, so that there isn't
a lot of visual noise when reading the function signatures.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Back when I wrote this code, I believe I set it to use a custom path
so that the cache would end up in a directory which Android considers
to be a cache directory. But nowadays the directory which Dolphin's
C++ code considers to be the cache directory is such a directory,
so there's no longer any reason to override the default path.

This is a missing feature from DolphinWX.

Deduplicates code, and gets rid of some problems the old code had
(such as: bad performance when calling native functions, only one
disc showing up for multi-disc games, Wii banners being low-res,
unnecessarily much effort being needed for adding more metadata).

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

DolphinWX already has this improvement in startup time, and it matters
a lot when you have a large game list.