Fix bug that caused methods to not be shown in the F6 dialog (find overrides).

The bug occurred when looking for the overrides of A.M() in this code:
  class A { virtual void M(); }
  class B : A {}
  class C : B { override void M(); }

C.M() was found, but not added to the tree because there isn't any intermediate node B.M().

src/Main/Base/Project/Src/Services/RefactoringService/ContextActions/ContextActionsHelper.cs

@@ -113,9 +113,21 @@ namespace ICSharpCode.SharpDevelop.Refactoring
 			
 			public ObservableCollection<ContextActionViewModel> BuildTreeViewModel(IEnumerable<ITreeNode<IClass>> classTree)
 			{
-				return new ObservableCollection<ContextActionViewModel>(
-					classTree.Select(
-						node => MakeGoToMemberAction(node.Content, BuildTreeViewModel(node.Children))).Where(action => action != null));
+				ObservableCollection<ContextActionViewModel> c = new ObservableCollection<ContextActionViewModel>();
+				foreach (var node in classTree) {
+					var childNodes = BuildTreeViewModel(node.Children);
+					var action = MakeGoToMemberAction(node.Content, childNodes);
+					if (action != null) {
+						c.Add(action);
+					} else {
+						// If the member doesn't exist in the derived class, directly append the
+						// children of that derived class here.
+						c.AddRange(childNodes);
+						// This is necessary so that the method C.M() is shown in the case
+						// "class A { virtual void M(); } class B : A {} class C : B { override void M(); }"
+					}
+				}
+				return c;
 			}
 		}
 	}