This could happen because insertContents() is called once for
every run of the content provider, even if an invalidation happens
in between. Example sequence:
    run()
    invalidate() [removes result of first run]
    run()
    insertContents() [queued for first run, retrieves result of second run]
    insertContents() [queued for second run, retrieves empty data]
We now check in insertContents() whether the content provider has a
valid root item and do nothing if it does not. This means that
insertContents() will never replace the current model data with empty
data; only invalidateContents() can do that from now on.
Further improvements:
    - Only call insertContents() if the run was not aborted; this
      reduces the number of useless objects in the event queue.
    - Remove the m_rootItem member; it was only used in the run() function.
    - Only add the root item to the list at the end of a successful run;
      there is no reason this object should be accessible from the outside
      while there are still children being added to it.

Change-Id: I80e2ea93dd9bbc8ab7f406c989b61f16f11b6eea
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>