Hide Objective-C from outside GdkQuartz.
Closes: #1737 (closed)
Don't export any functions taking or returning MacOS types in gdkquartz.h, gdkprivate-quartz.h, or any header that either includes.
The GdkQuartz internal functions are moved to a new header gdkinternal-quartz.h, the functions used by quartz-specific Gtk files are moved to another new header gdkquartz-gtk-only.h, and the key and event enums to a new header gdkkeys-quartz.h.