feat: Add keyboard back-light quick setting with slider

[3bnz]

output.mp4

• Adds a quick setting toggle with a slider in the menu for controlling built-in keyboard backlight.
• Supports a single keyboard - which should be fine since this does not concern bluetooth and usb peripherals.
• When toggling back-light off and on again the previous brightness value is retained
• Backlight symbol is not included i
  n nerd-fonts so I drew the missing icons and added them to AshellCustomIcon-Regular.otf

*I used fontforge to edit the font. You can see that after the edit - the size has decreased. Not sure why, but I did test that mouse and keyboard status bar icons still look as expected.

[romanstingler]

I like the feature, even if for some reason my HP is still not supported (not the issue of this here).
I will be honest, I don't like the UI.

In my opinion, we can use the same sliders as for the other components.
Or we can discuss wide sliders (like i.e. Android)

https://github.com/user-attachments/assets/abbe39c5-1fae-4333-aa6b-6810743300c5
demo at https://github.com/romanstingler/ashell/tree/feature/kbd ()no real kbd brightness, due to the lack of drivers supporting it for my system)

But a button that opens the slider is nothing I would like to have.