This formats all copyright comments according to SPDX formatting guidelines.
Additionally, this resolves the remaining GPLv2 only licensed files by relicensing them to GPLv2.0-or-later.

Creates a new BasicSettings class in common/settings, and forces setting
a default and label for each setting that uses it in common/settings.
Moves defaults and labels from both frontends into common settings.
Creates a helper function in each frontend to facillitate reading the
settings now with the new default and label properties.

Settings::Setting is also now a subclass of Settings::BasicSetting. Also
adds documentation for both Setting and BasicSetting.

- Removes a dependency on core and input_common from common.

Preliminary work for upmixing & general cleanup. Fixes basic issues in games such as Shovel Knight and slightly improves the LEGO games. Upmixing stitll needs to be implemented.

Audio levels in a few games will be fixed as we now use the downmix coefficients when possible instead of supplying our own

Gets rid of the largest set of mutable global state within the core.
This also paves a way for eliminating usages of GetInstance() on the
System class as a follow-up.

Note that no behavioral changes have been made, and this simply extracts
the functionality into a class. This also has the benefit of making
dependencies on the core timing functionality explicit within the
relevant interfaces.

These two macros being used in tandem were used prior to the
introduction of UNIMPLEMENTED and UNIMPLEMENTED_MSG. This provides
equivalent behavior, just with less typing/reading involved.

We can hide the direct array from external view and instead provide
functions to retrieve the necessary info. This has the benefit of
completely hiding the makeup of the SinkDetails structure from the rest
of the code.

Given that this makes the array hidden, we can also make the array
constexpr by altering the members slightly. This gets rid of several
static constructor calls related to std::vector and std::function.

Now we don't have heap allocations here that need to occur before the
program can even enter main(). It also has the benefit of saving a
little bit of heap space, but this doesn't matter too much, since the
savings in that regard are pretty tiny.