This also renamed some variables/functions.

The old tooltip description box used GraphicsWidget to provide shared
code to the Graphics config panes for adding descriptions to their
settings.

The description box has been replaced by BalloonTips and serves no
further purpose, so remove it and have the Graphics panes derive from
QWidget instead.

GraphicsRadioInt is used by the panes in the Graphics config window to
create radio buttons 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 ConfigRadioInt better reflects its
purpose. This should also make it less confusing when ConfigRadioInts
are added to other config panes.

m_xrr_config was used by the GeneralWidget::GetAvailableResolutions
function to get the list of supported fullscreen resolutions when
HAVE_XRANDR was set. aa4088a removed the ability to set that resolution
in the UI, leaving the GetAvailableResolutions function unused.

m_xrr_config is initialized in MainWindow which still uses it to toggle
fullscreen, but the references in GeneralWidget and GraphicsWidget
(which just ferried m_xrr_config from MainWindow to GeneralWidget) are
now unnecessary and removed in this commit.

GraphicsChoice is used by the panes in the Graphics config window to
create combo boxes 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 ConfigChoice better reflects its
purpose. This should also make it less confusing when ConfigChoices are
eventually added to the other config windows.

GraphicsBool is used by the panes in the Graphics config window to
create checkboxes 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 ConfigBool better reflects its
purpose. This should also make it less confusing when ConfigBools are
eventually added to the other config windows.

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.