Make it possible to gather values using scoped ItemConfiguration

Toolkit.Foundation/Observable.cs

@@ -1,4 +1,5 @@
 using CommunityToolkit.Mvvm.ComponentModel;
+using System.ComponentModel;
 
 namespace Toolkit.Foundation;
 
@@ -140,4 +141,11 @@ public partial class Observable<TKey, TValue> : Observable
         Key = key;
         Value = value;
     }
+
+    partial void OnValueChanged(TValue? value) => OnValueChanged();
+
+    protected virtual void OnValueChanged()
+    {
+
+    }
 }