Change internal class when attributes of properties change

Change internal class when attributes of properties change

This should allow us to share property attributes for different
class instances saving quite some memory. In addition, it
can be used to speed up property access (as we then know in the
lookup whether it's a data or accessor property).

Change-Id: Ide9c6168a07b5c83a1e73d075d8fc4f6594e08fd
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>