[FZ Editor] Limit zones number for custom layouts. (#9364)

* disable add zone button

* unify zone adding

src/modules/fancyzones/editor/FancyZonesEditor/MainWindow.xaml.cs

@@ -91,7 +91,7 @@ namespace FancyZonesEditor
                 return;
             }
 
-            if (model.TemplateZoneCount < LayoutSettings.MaxZones)
+            if (model.IsZoneAddingAllowed)
             {
                 model.TemplateZoneCount++;
             }