AllowedTypes attribute does not work with base classes

Steps to reproduce

When defining a base class for the AllowedTypes attribute, this is not handled out of the box for the client. The reason for this is that we do not send information about base classes to the client unless there is a specifically registered UIDescriptor for the base class.

How to test:

Add the following attribute to the StandardPage in the Alloy templates (either MVC or Web Forms, does not matter):

[AllowedTypes(new Type[] { typeof(SitePageData) })]

public virtual ContentArea MainContentArea { get; set; }

Make sure that all pages can be dragged into this content area except the root page.