Synchronize access to the EndpointReachedEvent listeners list.

I had a few of these exceptions while loading WPF UserControls:

```
System.Windows.Markup.XamlParseException: 'Loc:MyLabel_Text' value cannot be assigned to property 'Content' of object 'System.Windows.Controls.Label'. Collection was modified; enumeration operation may not execute.  Error at object 'WPFLocalizeExtension.Extensions.LocExtension' in markup file 'MyNamespace;component/folder/file.xaml' Line 20 Position 20. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at XAMLMarkupExtensions.Base.NestedMarkupExtension.EndpointReachedEvent.AddListener(NestedMarkupExtension listener)
   at XAMLMarkupExtensions.Base.NestedMarkupExtension.ProvideValue(IServiceProvider serviceProvider)
   at System.Windows.Markup.BamlRecordReader.ProvideValueFromMarkupExtension(MarkupExtension markupExtension, Object obj, Object member)
   at System.Windows.Markup.BamlRecordReader.SetDependencyComplexProperty(Object currentTarget, BamlAttributeInfoRecord attribInfo, Object o)
   --- End of inner exception stack trace ---
   at System.Windows.Markup.XamlParseException.ThrowException(String message, Exception innerException, Int32 lineNumber, Int32 linePosition, Uri baseUri, XamlObjectIds currentXamlObjectIds, XamlObjectIds contextXamlObjectIds, Type objectType)
   at System.Windows.Markup.XamlParseException.ThrowException(ParserContext parserContext, Int32 lineNumber, Int32 linePosition, String message, Exception innerException)
   at System.Windows.Markup.BamlRecordReader.ThrowExceptionWithLine(String message, Exception innerException)
   at System.Windows.Markup.BamlRecordReader.CreateInstanceFromType(Type type, Int16 typeId, Boolean throwOnFail)
   at System.Windows.Markup.BamlRecordReader.GetElementAndFlags(BamlElementStartRecord bamlElementStartRecord, Object& element, ReaderFlags& flags, Type& delayCreatedType, Int16& delayCreatedTypeId)
   at System.Windows.Markup.BamlRecordReader.BaseReadElementStartRecord(BamlElementStartRecord bamlElementRecord)
   at System.Windows.Markup.BamlRecordReader.ReadElementStartRecord(BamlElementStartRecord bamlElementRecord)
   at System.Windows.Markup.BamlRecordReader.ReadRecord(BamlRecord bamlRecord)
   at System.Windows.StyleHelper.LoadOptimizedTemplateContent(DependencyObject container, ParserContext parserContext, OptimizedTemplateContent optimizedTemplateContent, FrameworkTemplate frameworkTemplate, IComponentConnector componentConnector, IStyleConnector styleConnector, List`1 affectedChildren, UncommonField`1 templatedNonFeChildrenField)
   at System.Windows.FrameworkTemplate.LoadContent(DependencyObject container, List`1 affectedChildren, UncommonField`1 templatedNonFeChildrenField)
   at System.Windows.StyleHelper.ApplyTemplateContent(UncommonField`1 dataField, DependencyObject container, FrameworkElementFactory templateRoot, Int32 lastChildIndex, HybridDictionary childIndexFromChildID, FrameworkTemplate frameworkTemplate)
   at System.Windows.FrameworkTemplate.ApplyTemplateContent(UncommonField`1 templateDataField, FrameworkElement container)
   at System.Windows.FrameworkElement.ApplyTemplate()
   at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
   at System.Windows.UIElement.Measure(Size availableSize)
   at System.Windows.ContextLayoutManager.UpdateLayout()
   at System.Windows.ContextLayoutManager.UpdateLayoutCallback(Object arg)
   at System.Windows.Media.MediaContext.InvokeOnRenderCallback.DoWork()
   at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
   at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
   at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
```

They happened very sporadic and I could never reproduce them on my development machine. I also could not find out which thread modified the collection while enumerating it as it's a single threaded standard WPF application and every user interface modification happens through bindings.

However, this patch fixed all of them by simply synchronizing every access to the listeners list.