Cocoa: Get mouse position from the event

We want the coordinates for a mouse event to be the
position at the time of event generation, not the
current position.

This is a followup to 39be577c

 which fixed this for
some cases. This commit replaces all usages of
[NSEvent mouseLocation] with code that gets the location
from the event.

Change-Id: I691a46a61ce65e2b33335453b5b22de01f76d767
Task-id: QTBUG-54399
Task-id: QTBUG-37926
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>