Refactor GetPropertyValueType

[zpqrtbnk]

Imagine a picker that picks content type alias "foo" - we want GetPropertyValueType to return typeof(Models.Foo) but that is not possible because the type does not exist because it is being generated. So, GetPropertyValueType needs to be refactored to return a string and be passed a dictionary of (alias, type name).

[zpqrtbnk]

This is now provided by the ModelType class in Umbraco.Core, used for instance in the nested content value converters - closing.