Hide Objective-C from outside GdkQuartz.

Closes: GNOME/gtk#1737

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.
2 jobs for no-objc in 6 minutes and 26 seconds (queued for 2 seconds)
latest
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passed #247182
build

00:06:26

manual #247183
win32 allowed to fail manual
msys2-mingw32