Hide Objective-C from outside GdkQuartz.

Closes: https://gitlab.gnome.org/GNOME/gtk/issues/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.

gdk/quartz/gdkmonitor-quartz.c

@@ -23,7 +23,6 @@
 #include "gdkmonitor-quartz.h"
 #include "gdkdisplay-quartz.h"
 
-
 G_DEFINE_TYPE (GdkQuartzMonitor, gdk_quartz_monitor, GDK_TYPE_MONITOR)
 
 static void