[osx] - add function for identifying osx snow leopard runtime
[settings] - add settingscondition OsxIsSnowLeopard
[osx/settings] - make the fake fullscreen option only visible when running
on snowleopard - it leads to black screen on all later osx versions
Merge pull request #4922 from Memphiz/osxfakefullscreenbp

[osx] - only show the fake fullscreen option on snow leopard
Merge branch 'Gotham' of https://github.com/xbmc/xbmc into Gotham