Improve slider control: enable incremental adjustments via clicking and scrolling.

[gileli121]

Based on what I observed, there are two issues:

• Clicking on a specific point on the slider causes it to jump to 0% or 100% instead of jumping to the correct percentage based on the click position.
• Scrolling with the mouse wheel over the slider does nothing.

User report

Adjusting the slider is a bit clunky. Clicking on it makes it jump all the way from one end to the other, instead of jumping 5 or 10% as would be expected, and it can't be scrolled, so the only way to adjust it is by dragging it. Clicking and scrolling should both jump it by a certain amount, preferably customized in settings along with the current option for adjusting the scroll sensitivity when scrolling while holding a modifier key.

Originally reported in
#415