Ensure that child MouseAreas will get the event first

By lowering the MouseArea provided by a ScrollView we can ensure that
any user MouseArea created as a child will get the event first which can
pass it on afterwards.

Task-number: QTBUG-38083

Change-Id: Id0a36fab7600966a0e12043fa7e8a98641d71a81
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>