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.

fix some related grammar errors
only the ButtonManager required code changes

Makes it less error-prone to get state data from sliders (no need
to pass any locals), and also allows direct assignment, letting the
retrieved data be const.

Same as the previous commit, except I'm copying strings
in the other direction because the DolphinWX variants
of these strings could use some improvement.

Replaces includes with forward declarations where applicable, and moves
includes to where they're actually needed.

ControllerEmu, the class, is essentially acting like a namespace for
ControlGroup. This makes it impossible to forward declare any of the
internals. It also globs a bunch of classes together which is kind of a
pain to manage.

This splits ControlGroup and the classes it contains into their own source
files and situates them all within a namespace, which gets them out of
global scope.

Since this allows forward declarations for the once-internal classes, it
now requires significantly less files to be rebuilt if anything is changed
in the ControllerEmu portion of code.

It does not split out the settings classes yet, however, as it
would be preferable to make a settings base class that all settings derive
from, but this would be a functional change -- this commit only intends to
move around existing code. Extracting the settings class will be done in
another commit.