Random noise filter improvements

• Add "grayscale" option to the random noise filter
• Increase the sliders' max value from 99 to 100.
• Improve the performance (I used fast color overlay filter as the example)

This MR adds the following feature request: https://krita-artists.org/t/monochromatic-noise-feature/113718/

Basically this adds the grayscale checkbox to the widget. As for the max sliders, it seemed like a bug to me. If the slider caps at 99, then we will never achieve 100% coverage or opacity. I spent some extra time to clean up the code and improve the performance (on my system it seems to be at least twice as fast now).

Here's an example of it in action: