Some applications need to access gdk_quartz_window_get_nsview,
gdk_quartz_window_get_nswindow, and gdk_quartz_event_get_nsevent
so move these from the private gdkquartz-gtk-only.h to a new
header gdkquartz-cocoa-access.h. Don't include this header in
gdkquartz.h so that user code that doesn't need to access these
functins isn't required to compile with Objective C/C++.
Closes https://gitlab.gnome.org/GNOME/gtk/-/issues/1737
Current problems:
* other widgets in a GL-painted window are low-resolution on Retina
display
* something wrong with paint updates; gdkgears demo only updates every
couple of seconds but reports ~30fps
See: #517
