Trackpad sensitivity for knobs and spinboxes

See also:
* #5360 (knobs moving too fast)
* #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.

Start by locating the LCD Spinbox, knob, and master volume slider implementations. Compare the trackpad or two-finger scroll handling with the QAbstractSlider behavior mentioned in the issue, then reproduce the sensitivity difference on supported platforms. Done means trackpad gestures move spinboxes less aggressively and knobs more responsively while the master volume slider remains unchanged.

Written by the indexing model from the issue text.