OS X: Use Application Support directory as user data path

Source: http://forum.openmw.org/viewtopic.php?f=20&t=2677&start=30#p30706

This change is kinda breaking because users will have to move their saves and screenshots from Preferences to Application Support after OpenMW update.

(RM-2460 from redmine: created on 2015-03-18 by Nikolay Kasyanov, , closed on 2015-03-19 by Nikolay Kasyanov)