Make ViewModel.OnStateChanged public

for now, while we are not restoring state automatically

src/BitzArt.Blazor.MVVM/Models/ViewModel.cs

@@ -72,5 +72,5 @@ public virtual void OnStateRestored() { }
     /// <summary>
     /// Called when the state has changed.
     /// </summary>
-    protected virtual void OnStateChanged(TState state) { }
+    public virtual void OnStateChanged(TState state) { }
 }