Make objects inside properties accessible.

Similar to the support for factory-methods, we must wrap objects in
properties to make them accessible to clients. This patch adds the
required code for that. Besides support for simple properties that
reference an object, this patch also adds support for list properties
that contain objects.

The client-side unwrap of properties is delayed until all objects
are initialized, as a property might reference another registered
object.

Change-Id: I9fb90a8eab4c66d2f4231fdb482e0d97d128df3e
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>