Fix handling of high precision scroll events

While Konsole's main scrollbar is handled by QtScrollBar and therefore
works as expected under Qt5, the other custom scroll wheel actions
available do not accumulate and handle the small scroll deltas provided.
This results in stuttery, uneven and unexpected responses to scrolling.

Accumulate and trigger events on thresholds. If pixel scroll values are
provided, utilize them when emulating mouseup/mousedown events (by
dividing by pixel font height).

Patch by Lindsay Roberts linds r gmail com

REVIEW: 124590