warn serializing nested classes derived from UEComponent

[extrawurst]

issue warning for serializing a member that inherits from UEComponent if that class is nested but the member is of a type up the herarchy. use http://dlang.org/traits.html#isNested

rationale: the class name is used on deserialization to create that member using Object.factory but that wont work for nested types (even when only nested in a unittest block ^^)