quartz: implement gdk_window_set_shadow_width()

Use the information to allow dragging windows all the way to the top of
the screen (ie: allow the top shadow to go under the menubar).

https://bugzilla.gnome.org/show_bug.cgi?id=720374

gdk/quartz/gdkwindow-quartz.h

@@ -62,6 +62,8 @@ struct _GdkWindowImplQuartz
   cairo_region_t *needs_display_region;
 
   cairo_surface_t *cairo_surface;
+
+  gint shadow_top;
 };
  
 struct _GdkWindowImplQuartzClass