Separate slider for overlay opacity

[julienfalque]

Disabling "Use backlight control" option is great if one wants to be able to control backlight brightness and overlay opacity separately. Though it also changes the behavior of Gnome's brightness slider so that it controls overlay opacity instead of backlight brightness, which forces to use hotkeys to change the backlight brightness. I would like the slider behavior to remain unchanged, would it be possible to add a second slider that controls overlay opacity?

[F-i-f]

Using both the backlight and the overlay at the same time and controlling them separately wasn't in my use cases.

I'm wondering if this new mode of operation would be covered by the existing settings, or if a new setting is needed.

I'd assume if Use backlight control is off and Monitor(s) is set to All, then any existing brightness slider could be left alone.

I may add this feature, but it's not very high in my to-do list. Pull Requests welcome.