Revert "Use onEntered in KickoffItemDelegate"

This reverts commit 8042ae81.

This change slightly improved performance, but subtle implementation
differences between the onEntered: and onPositionChanged: handlers in
MouseArea triggered two subtle and annoying bugs that users have
reported a bunch of duplicates of.

A way to fix the bugs using onEntered has not been found, so let's revert
the change for now; slightly worse performance is a less severe issue then
these bugs are.

BUG: 454349
BUG: 455674
FIXED-IN: 5.25.5