Use mouse instead of arguments[0] in signal handlers

Couldn't reproduce the bug that was the explained cause and based on the bug
comments it hasn't been reproducible in Qt 5.2+ in 2014 either.

Task-number: QTBUG-36560
Change-Id: I198bbba5a3f00bf49a7efbfde30af1696011efd8
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>