Add preference for mousemove sensitivity during window leveling

[btbest]

Inspiration: https://forum.image.sc/t/adjusting-image-brightness-contrast-and-histogram-to-color-mapping-in-training/111154/3

I've also personally never used the window leveling tool because it's far too finicky. It would be nice at least as an advanced option to make it possible to change the sensitivity in the preferences.json, and maybe a lower default value would generally be good.

As far as I can tell, the sensitivity of the tool (internally called Thresholding) is defined by ThresholdingInterpreter._steps_scaling and is hard-coded.

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/adjusting-image-brightness-contrast-and-histogram-to-color-mapping-in-training/111154/4