Move brightness and contrast together

[lucivpav]

Would you support to include a new feature that, when enabled, would allow user to drag both the brightness and contrast simultaneously? Inspired by EIZO monitors and also MacBook internal screen that have a single intensity slider, which is really easy and convenient to use.

I'd say that typically abrubt changes to brightness have to also have change in contrast. And vice versa. This feature would simplify the process. After the mutual drag, user could still tweak the set-up by slightly adjusting one of the sliders.

[emoacht]

In such case, I think the conditional commands where the change of the brightness will trigger to change the contast would be useful. You will need to prepare 100 commands for 100 brightness levels in total though.
See #563