For the various uses of GDK_WINDOWING_QUARTZ, we need to use
alternatives from GDK_WINDOWING_MACOS.
Some minor loss of functionality is here, such as icons sent with
application menus. That can certainly be added back at a future
point.
https://bugzilla.gnome.org/show_bug.cgi?id=784723 introduced support for
native file chooser dialogs on macOS, but due to the use of generics in
the patch, there will be compilation errors on pre-Xcode 7 platforms,
such as Mountain Lion and Mavericks.
I strongly recommend to revert this patch when the oldest supported
macOS release is bumped to Yosemite (10.10).
https://bugzilla.gnome.org/show_bug.cgi?id=785306
