kcms/mouse: add ContextualHelpButton for on-button scrolling
Since Plasma 6.2, we allow users to toggle the libinput on-button scrolling option from the mouse kcm. This function is intended for scroll-wheel emulation on devices that do not have one, such as pointing sticks, although some users may enjoy it on other devices as well. Users often come looking for something more akin to Windows' auto-scrolling feature, which we do not support. This leads to confusion, as its behavior is substantially different from the feature they are looking for, and there are significant downsides for some applications to having this enabled (as it disables all middle-click dragging functionality).
Let's add a helper button that makes clear (a) what the downside of enabling this option is and (b) what it is primarily intended for, so users don't think it's a botched implementation of auto-scroll.