Convert to WeakReference in ExpressionObserver.

Doing a `Publish().Refcount()` caches the latest value so it can be sent
to subsequent subscribers, and this causes a leak. Convert the value
to/from `WeakReference` for that part.

src/Markup/Avalonia.Markup/Data/ExpressionObserver.cs

@@ -177,9 +177,11 @@ namespace Avalonia.Markup.Data
                 }
 
                 _result = Observable.Using(StartRoot, _ => source)
+                    .Select(ToWeakReference)
                     .Publish(UninitializedValue)
                     .RefCount()
-                    .Where(x => x != UninitializedValue);
+                    .Where(x => x != UninitializedValue)
+                    .Select(FromWeakReference);
             }
 
             return _result.Subscribe(observer);
@@ -197,6 +199,16 @@ namespace Avalonia.Markup.Data
             }
         }
 
+        private static object ToWeakReference(object o)
+        {
+            return o is BindingNotification ? o : new WeakReference(o);
+        }
+
+        private static object FromWeakReference(object o)
+        {
+            return o is WeakReference ? ((WeakReference)o).Target : o;
+        }
+
         private IDisposable StartRoot()
         {
             var observable = _root as IObservable<object>;