Fix parts of workerscript

Fix parts of workerscript

The worker script creates a special QmlContextWrapper
that doesn't contain many things found in the main thread.
However we still need to be able to cast to it, so it
should be the same class as the regular context wrapper.

Fixes parts of the worker script auto tests.

Change-Id: I3697b2b0080dc4ac967eb447e2efd0f28fbab465
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>