Avoid adding the extension element if the user did not change anything.

Author: dgrunwald

src/AddIns/BackendBindings/CSharpBinding/Project/Src/Project/CSharpProject.cs

@@ -142,9 +142,11 @@ public override void ProjectLoaded()
 		public override void Save(string fileName)
 		{
 			// Save project extensions
-			var propertiesXElement = extensionProperties.Save();
-			SaveProjectExtensions(ExtensionPropertiesName, propertiesXElement);
-			
+			if (extensionProperties.IsDirty) {
+				var propertiesXElement = extensionProperties.Save();
+				SaveProjectExtensions(ExtensionPropertiesName, propertiesXElement);
+				extensionProperties.IsDirty = false;
+			}
 			base.Save(fileName);
 		}