* Revised TSnippetValidator.Validate to call ValidateExtra.
* Reimplemented and shortened TSnippetValidator's HasValidDependsList overloaded methods, removing duplicated code. Renamed the methods as ValidateDependsList for consistency with other methods.
* Added new TSnippetValidator.ValidDependsKinds method that returns a set of snippet kinds that may be included in the dependency list of a specified snippet kind.
