Properly support the new-style built-in full screen mode of Mac OS X (version 10.7 and later)

Submitted by Kristian Rietveld @kristian

Link to original bug (#765163)

Description

We need support in GTK+ for the fullscreen mode that is found in OS X versions 10.7 and later. If this support is there, we need to make GIMP play well with this and then remove the currently "old-style" fullscreen mode available through "View > Fullscreen".

Version: git master