Draft: kcms/mouse,touchpad: make natural scrolling consistent and explain

The Mouse and Touchpad kcms use different labels for the natural scrolling option, and both emphasize that it inverts the scrolling direction. But this is not actually true; it applies only to views, but not to sliders and similar controls. This occasionally confuses users who expect the setting to actually invert scrolling everywhere, but the intent is to provide natural scrolling as on a touchscreen, or as if directly interacting with the object in question.

This change labels both options "Natural scrolling" and adds contextual help buttons to explain what this means.