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.

Since all FS access will go through the new FS interface (PR #6421)
in order to keep track of metadata properly, there is no need to return
absolute paths anymore.

In fact, returning host paths is a roadblock to using the FS interface.

This starts the migration work by adding a way to get paths that are
relative to the Wii NAND instead of always getting absolute paths
on the host FS.

To prepare for future changes, this commit also makes returned paths
canonical by removing the trailing slash when it's unneeded.

Eventually, once everything has been migrated to the new interface,
we can remove the "from" parameter.