Introduced NamespaceDeclaration.IdentifierTokens (for MonoDevelop compatibility)

Commit dd43d89be81141a602e3a99c1fda808da2d86933 made Identifiers return an `IEnumerable<string>`, but some MonoDevelop code still expected `IEnumerable<Identifier>`, so I created a new property for that.

See also https://github.com/mono/monodevelop/pull/315