Add Undetermined value as default of RefactoringDialogResult

This fixes that the default dialog reult was Execute, which caused refactorings to execute if the corresponding dialog was closed by clsing the window.

Author: MDoerner

Rubberduck.Refactorings/IRefactoringDialog.cs

@@ -4,6 +4,7 @@ namespace Rubberduck.Refactorings
 {
     public enum RefactoringDialogResult
     {
+        Undetermined,
         Execute,
         Cancel
     }