[BUG] Selection a position for a new page doesn't work if there are versions of any page at the same level of the tree

[Artur0802]

Scenario:

1. Login to C1 CMS Console.

2. Install Orckestra.Versioning.VersionPublication addon if it's not installed.

3. Create a version of any child page in the tree.

   3.a. Select any child page in the tree and click "Edit page":

 3.b. Click "Add version" at the opened form:

 3.c. Enter the name of the new version and click "OK":

4. Return to the "Content" tab and start adding a new page at the same level of the tree:

5. Enter the title and choose the "Select position" option in the "Position field":

6. Click "Next" and the error will occur:

The full text of the error is attached - Error.txt

Looks like there is a related to the Orckestra.Versioning.VersionPublication​ addon bug in this class (https://github.com/Orckestra/C1-CMS-Foundation/blob/dev/Composite.Workflows/Plugins/Elements/ElementProviders/PageElementProvider/AddNewPageWorkflow.cs) .
If we choose "Select position" option we get into this block of code:

if (this.GetBinding<string>("SelectedSortOrder") == SortOrder.Relative)
            {
                Dictionary<Guid, string> existingPages = PageServices.GetChildren(GetParentId()).ToDictionary(page => page.Id, page => page.Title);

                this.Bindings["ExistingPages"] = existingPages;
                this.Bindings["RelativeSelectedPageId"] = existingPages.First().Key;
            }

The problem is PageServices.GetChildren(GetParentId()) method returns all versions of a page. As all versions of the same page have the same PageId we get the error.