Support changing screen brightness on desktops

[bradfitz]

Expected behaviour

I'd like the Power Manager Brightness Applet to also be able to adjust desktop monitor brightness.

Actual behaviour

I add the brightness applet to my panel, but it has a red cross through it.

Presumably it doesn't work because I don't have a backlight class on this desktop machine:

$ ls -l /sys/class/backlight/
total 0

But xrandr works fine:

$ xrandr --output DP-1 --brightness 0.6

It'd be nice if the brightness applet could fall back to doing whatever xrand's doing, even if it just means shelling out to xrandr.

Steps to reproduce the behaviour

Install Brightness applet

MATE general version

1.24.0-2

Package version

1.24.1

Linux Distribution

Ubuntu Focal Fossa

Link to downstream report of your Distribution

None.