Trackpad sensitivity for knobs and spinboxes

[tresf]

See also:

• Fix #5194: Fix knobs moving too fast #5360 (knobs moving too fast)
• Natural scrolling is backwards for knobs, spinboxes #5507 (natural scrolling is backwards)

When using a trackpad to "scroll" (e.g. a two-finger gesture -- supported by Windows, Linux and Mac), LCD Spinboxes are too sensitive. Knobs aren't sensitive enough.

Note, the master volume slider feels OK. According to @PhysSong this is because it uses QAbstractSlider. Perhaps we could borrow some sensitivy logic from there.