Unlike custom banners which work as an override, this mechanism works as
a fallback. The use case is if you have games you don't really play but
want to keep around for testing purposes without filling up your NAND
with lots of saves. For ease of use, the directory structure is the same
but only title/$title_hi/$title_lo/data/banner.bin files are
relevant.

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.

Since C++17, non-member std::size() is present in the standard library
which also operates on regular C arrays. Given that, we can just replace
usages of ArraySize with that where applicable.

In many cases, we can just change the actual C array ArraySize() was
called on into a std::array and just use its .size() member function
instead.

In some other cases, we can collapse the loops they were used in, into a
ranged-for loop, eliminating the need for en explicit bounds query.

Makes the function names conform to our coding style.

Given this is within Common, it should be in the Common namespace
itself.

Fixes https://bugs.dolphin-emu.org/issues/10938
and makes PNG banners available in DolphinQt2 for the first time.