Make it possible to run a nested task tree containing
the common storage from the outer task tree's handler.
This means we need to manage the stack of active storages
per thread.
Add a test for it.
This addresses the 4th point in the report below.
Task-number: QTCREATORBUG-29103
Change-Id: Ib0f925048197625a0eb9f83f262c7256a5756cc8
Reviewed-by: hjk <hjk@qt.io>