Fix Hide XPath Results menu option.

ITextMarkerService is not returned by IViewContent.GetService()
but is available from ITextEditor.

src/AddIns/DisplayBindings/XmlEditor/Project/Src/RemoveXPathHighlightingCommand.cs

@@ -32,7 +32,10 @@ namespace ICSharpCode.XmlEditor
 		public void RemoveXPathNodeTextMarkers()
 		{
 			foreach (IViewContent view in workbench.ViewContentCollection) {
-				XPathNodeTextMarker.RemoveMarkers(view);
+				ITextEditor textEditor = view.GetService<ITextEditor>();
+				if (textEditor != null) {
+					XPathNodeTextMarker.RemoveMarkers(textEditor);
+				}
 			}
 		}
 	}

src/AddIns/DisplayBindings/XmlEditor/Test/Utils/MockTextEditor.cs

@@ -179,6 +179,9 @@ namespace XmlEditor.Tests.Utils
 		
 		public object GetService(Type serviceType)
 		{
+			if (serviceType == typeof(ITextEditor)) {
+				return this;
+			}
 			return document.GetService(serviceType);
 		}