FR: modify a percentage exponentially

[wiiznokes]

It's really annoying on my laptop, because the minimal luminosity is too low, and the minimal luminosity + 1 is too high.

I would really prefer if the click on the brightness button start by increasing by one or two. And incremental click will up this delta. Let's say 1, then 2, then 3, 4, and 5.

[ids1024]

I've noticed this. It seems like it would make sense to apply a logarithmic scale for brightness steps.

Though what I wonder is if this is consistent across backlight devices, or if some could use a different scale. And if there's a way to tell.

[ids1024]

For backlights that work this way, it would also make sense to use a log scale for the brightness slider. But still, not sure if all devices work the same way, or if some might be scaled like this already.

The currently implementation attempts to match what gnome-settings-daemon does, so it's at least been proven to work well enough most of the time.