This fixes an issue where the game specific graphics backend would be saved as the global setting after playing a game.

This also now displays the currently running graphics backend when looking in the graphics configuration window.

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.

These variables are only used in CreateMainLayout and can thus be local
instead.

Thus, the software renderer uses the same settings as everything else, fixing https://bugs.dolphin-emu.org/issues/13051.

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.

Fixes https://bugs.dolphin-emu.org/issues/12087

The current approach results in the UI thread creating a graphics device
whilst the core is running, leading to races on function pointers, and
potentially crashing.