Add option to limit lowest brightness for software dimming

[waydabber]

Discussed in #2982

^{Originally posted by I3HowA May 17, 2024}
Hi,
I came from Monitor Control, there's an option to prevent software dimming to 0% brightness (total black).
I'm wondering if BetterDisplay have similar setting to limit the lowest possible brightness for software adjustment.
I see the option to set minimum hardware brightness, but I couldn't find anything to limit software dimming.
If I somehow make it 0% by mistake, it'll be pretty hard to locate the slider in complete darkness.

Thanks