TopicMappingService: Improve IsVisible() support

[JeremyCaney]

A previous commit, #885d9a6d, introduced a bypass for IsVisible() when adding topics to a collection via TopicMappingService. This introduced problems since some customers have a requirement to provide an index of topics that are hidden from the navigation using IsHidden. For that reason, this changed was rolled back in #cccf94c.

Ideally, we will reintroduce this with a more careful implementation. This should include:

• A ValidateTopic() function to centralize checks for null, IsDisabled, IsHidden, and List.
• Possibly refactoring of PopulateChildTopics() since it is no longer a primary entry point.
• A [AllowHidden] attribute on properties to bypass the check for IsHidden.