Protect the creation of the static GtkOSXApplication pointer with a GStaticMutex for thread safety. 
Protect the creation of the static GtkOSXApplication pointer with a GStaticMutex for thread safety. 

(The GOnce version was misguided; GOnce is to protect thread creation objects, not singletons.)
jralls/gtk-mac-integration,sharoonthomas/gtk-mac-integration,GNOME/gtk-mac-integration,jralls/gtk-mac-integration,sharoonthomas/gtk-mac-integration,jralls/gtk-mac-integration,sharoonthomas/gtk-mac-integration,sharoonthomas/gtk-mac-integration,GNOME/gtk-mac-integration,GNOME/gtk-mac-integration,sharoonthomas/gtk-mac-integration