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/gdkquartz-gtk-only.h

@@ -0,0 +1,50 @@
+/* gdkquartz-gtk-only.h
+ *
+ * Copyright (C) 2005-2007 Imendio AB
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GDK_QUARTZ_GTK_ONLY_H__
+#define __GDK_QUARTZ_GTK_ONLY_H__
+
+#if !(defined (GTK_COMPILATION) || defined (GDK_COMPILATION))
+#error "This API is for use only in Gtk internal code."
+#endif
+
+#include <Appkit/Appkit.h>
+#include <gdk/gdk.h>
+#include <gdk/quartz/gdkquartz.h>
+
+/* Drag and Drop/Clipboard */
+GDK_AVAILABLE_IN_ALL
+GdkAtom   gdk_quartz_pasteboard_type_to_atom_libgtk_only        (NSString       *type);
+GDK_AVAILABLE_IN_ALL
+NSString *gdk_quartz_target_to_pasteboard_type_libgtk_only      (const gchar    *target);
+GDK_AVAILABLE_IN_ALL
+NSString *gdk_quartz_atom_to_pasteboard_type_libgtk_only        (GdkAtom         atom);
+
+/* Utilities */
+GDK_AVAILABLE_IN_ALL
+NSImage  *gdk_quartz_pixbuf_to_ns_image_libgtk_only (GdkPixbuf *pixbuf);
+GDK_AVAILABLE_IN_ALL
+NSEvent  *gdk_quartz_event_get_nsevent              (GdkEvent  *event);
+
+/* Window */
+GDK_AVAILABLE_IN_ALL
+NSWindow *gdk_quartz_window_get_nswindow            (GdkWindow *window);
+GDK_AVAILABLE_IN_ALL
+NSView   *gdk_quartz_window_get_nsview              (GdkWindow *window);
+
+#endif