Cocoa: Send proper mouse coordinates.

Cocoa: Send proper mouse coordinates.

Clear up coordinate system confusion. Also change
QApplicationPrivate::pickMouseReceiver to indicate
that, yes, it does modify the 3rd argument in
addition to returning the mouse receiver widget.

Change-Id: If2c44eff65aedfdc78c6da1728f31d6e2db71f9d
Reviewed-on: http://codereview.qt-project.org/5919
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@nokia.com>