Add a public header for Cocoa access functions.
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 #1737 (closed)