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.

Moves it closer to where its used, narrowing its visible scope, as well
as preventing unnecessary std::string constructor executions in the
event invalid data is encountered (the continue branch).

std::function is allowed to heap allocate in order to store captured
variables, etc, so std::function isn't a trivial type. We can std::move
here in order to avoid potential reallocating.

While we're at it, make the definition's parameter name match the
declaration's parameter name for consistency.

If the parameter is const, then a move won't actually be able to occur,
making the std::move non-functional. We can remove the const qualifier
to remedy this.