Swapped names in ExpressionNode.

Swapped names of StartListening and StartListeningCore - method names
ending with Core should be the subclass implementation.

src/Markup/Avalonia.Markup/Data/EmptyExpressionNode.cs

@@ -10,7 +10,7 @@ namespace Avalonia.Markup.Data
     {
         public override string Description => ".";
 
-        protected override IObservable<object> StartListening(WeakReference reference)
+        protected override IObservable<object> StartListeningCore(WeakReference reference)
         {
             return Observable.Return(reference.Target);
         }

src/Markup/Avalonia.Markup/Data/ExpressionNode.cs

@@ -42,7 +42,7 @@ namespace Avalonia.Markup.Data
 
                     if (running)
                     {
-                        _valueSubscription = StartListeningCore();
+                        _valueSubscription = StartListening();
                     }
                 }
             }
@@ -57,7 +57,7 @@ namespace Avalonia.Markup.Data
 
             _observer = observer;
             var nextSubscription = Next?.Subscribe(this);
-            _valueSubscription = StartListeningCore();
+            _valueSubscription = StartListening();
 
             return Disposable.Create(() =>
             {
@@ -85,7 +85,7 @@ namespace Avalonia.Markup.Data
             NextValueChanged(value);
         }
 
-        protected virtual IObservable<object> StartListening(WeakReference reference)
+        protected virtual IObservable<object> StartListeningCore(WeakReference reference)
         {
             return Observable.Return(reference.Target);
         }
@@ -97,7 +97,7 @@ namespace Avalonia.Markup.Data
             _observer.OnNext(value);
         }
 
-        private IDisposable StartListeningCore()
+        private IDisposable StartListening()
         {
             var target = _target.Target;
             IObservable<object> source;
@@ -112,7 +112,7 @@ namespace Avalonia.Markup.Data
             }
             else
             {
-                source = StartListening(_target);
+                source = StartListeningCore(_target);
             }
 
             return source.Subscribe(TargetValueChanged);

src/Markup/Avalonia.Markup/Data/IndexerNode.cs

@@ -23,7 +23,7 @@ namespace Avalonia.Markup.Data
 
         public override string Description => "[" + string.Join(",", Arguments) + "]";
 
-        protected override IObservable<object> StartListening(WeakReference reference)
+        protected override IObservable<object> StartListeningCore(WeakReference reference)
         {
             var target = reference.Target;
             var incc = target as INotifyCollectionChanged;

src/Markup/Avalonia.Markup/Data/PropertyAccessorNode.cs

@@ -35,7 +35,7 @@ namespace Avalonia.Markup.Data
             return false;
         }
 
-        protected override IObservable<object> StartListening(WeakReference reference)
+        protected override IObservable<object> StartListeningCore(WeakReference reference)
         {
             var plugin = ExpressionObserver.PropertyAccessors.FirstOrDefault(x => x.Match(reference));
             var accessor = plugin?.Start(reference, PropertyName);