Support serializing nested and generic private types.
The string-to-type resolver does currently not support resolving types that are private and/or nested and/or generic. The XML serializer/deserializer usually uses that the type is known on deserialzation and assumes the type is the same as the property unless otherwise specified. However, if the property type is e.g an IList but the instance is a PrivateList, then currently the resolver is not able to locate the type, even though it might be something simple like, MyPlugin.PrivateList.
I suggest we add an attribute to highlight private types that might be needed during deserialization.