applets/taskmanager: turn off wrapping while scrolling

Currently, when the user scrolls past the beginning or end of the tasks represented in the task manager, it wraps around and continues from the other side.

This does not conform to the convention that exists for tabs (essentially similar UI elements), namely that keyboard shortcuts wrap but scrolling on the task bar does not, as can be seen in Dolphin, Konsole, or in non-KDE applications, Firefox. (Kate seems to never wrap, whether with keyboard shortcuts or scrolling). Not wrapping when scrolling is particularly helpful for users of touchpads, non-clicky-wheel mice, trackpoints/pointing sticks, and (I assume) trackballs, where it is very easy to overshoot when scrolling larger distances. See BUG: 404781 and BUG: 489132 for users requesting this behavior.

This MR makes the task manager not wrap when scrolling with the mouse wheel. The MR also contains a minor refactoring of parts of the JS code, mainly to keep the lines short and readable by removing some repetition.

@teams/vdg

BUG: 404781
FIXED-IN: 6.2.0