GraphicsSlider is used by the panes in the Graphics config window to
create sliders that change their associated config setting, and update
their own state when something else changes the config setting.
Despite its current name nothing about this class is particular to the
Graphics window, so renaming it to ConfigSlider better reflects its
purpose. This should also make it less confusing when ConfigSliders are
added to other config panes.
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.
