Disable the NS app when no frames are left (bug#14619, bug#21357, bug#23586)

* src/nsfns.m (Fx_create_frame): Enable the app.
* src/nsterm.m (ns_delete_terminal): Disable the app.

src/nsfns.m

@@ -1343,6 +1343,11 @@ DEFUN ("x-create-frame", Fx_create_frame, Sx_create_frame,
 
   f->output_data.ns->in_animation = NO;
 
+#ifdef NS_IMPL_COCOA
+  /* If the app has previously been disabled, start it up again.  */
+  [NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];
+#endif
+
   [[EmacsView alloc] initFrameFromEmacs: f];
 
   ns_icon (f, parms);

src/nsterm.m

@@ -5014,6 +5014,12 @@ ns_delete_terminal (struct terminal *terminal)
 
   block_input ();
 
+#ifdef NS_IMPL_COCOA
+  /* Rather than try to clean up the NS environment we can just
+     disable the app and leave it waiting for any new frames.  */
+  [NSApp setActivationPolicy:NSApplicationActivationPolicyProhibited];
+#endif
+
   image_destroy_all_bitmaps (dpyinfo);
   ns_delete_display (dpyinfo);
   unblock_input ();