Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

Fixes a crash that could occur if the static constructor function for
the MainSettings.cpp TU happened to run before the variables in
Common/Version.cpp are initialised. (This is known as the static
initialisation order fiasco.)

By using wrapper functions, those variables are now guaranteed to be
constructed on first use.

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.

This variable isn't std::moved anywhere and is just read out of into a
string. Instead of making a copy, and then another copy of the data into
a std::string, we can take it by reference, only copying the data once.

These parameters only acted as views into the provided strings, so these
can just be turned into non-owning string views.

This isn't std::moved wholesale into a member variable or further
std::moved into another function, so it's better to take it by const
reference here to avoid unnecessary reallocations of contained
std::string instances.