Move the internalClass pointer from Object to Managed

Move the internalClass pointer from Object to Managed

This allows us to get back the ExecutionEngine for all
memeory managed objects, so we can avoid lots of special
handling for String objects.

Simplify and cleanup QV4::PersistentValue and make use
of the available engine pointer in Managed.

Change-Id: Ia2e08b6207f7e706c23be219d5f6e74b414035a3
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>