Hardware brightness control

[Massimo-B]

Current version: 2.4

As described in https://gitlab.xfce.org/xfce/xfce4-power-manager/-/issues/158#note_94788 your software only controls the software gamma levels and not the hardware brightness, so it's no solution for me.

Hardware is controlled for the internal display via

# grep -r ^ /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/*brightness
/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/actual_brightness:19200
/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/brightness:19200
/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/max_brightness:19200

or for the external HDMI display via I2C using ddcui (https://www.ddcutil.com/ddcui_main/) or gddccontrol/ddccontrol (https://github.com/ddccontrol/ddccontrol/), affecting directly the brightness in its internal setup menu.

None of the tools like Xfce Power Manager Plugin (only supporting the internal display) or the mentioned gddccontrol/ddccontrol (external displays via I2C only) is able to control all kind of displays in one application.

Could you add hardware brightness support?