a11y: Top-level item views don't have the application as accessible parent

While other top-level widgets (e.g. QWidget) correctly have the application as a parent in the accessible tree, item views (like QTreeView)
that are created without a widget as parent do not report any parent.
The application object still lists them as children, so the accessible tree is apparently incorrect.

The attached sample program (based on the flattreeview example removed in ce13dc8c2f73408ce2897f4bd044560955e38145 ) can be used to reproduce this. It hits an assert that shows the issue.

Steps to reproduce:

1) build the attached sample program

2) run it

Actual result: an assert is triggered

Expected result: The program should run fine.

Alternative way to reproduce/analyze:

After removing the asserts, the issue can also be manually inspected by e.g. using Accerciser (on Linux) to inspect the accessible tree.

With the application object selected in accerciser's a11y hierarchy, typing the following in its IPython console shows that the application has the item view as a child at index 1, but that child does not have any parent set:

In [1]: acc.name
Out[1]: 'flattreeview'
In [2]: acc.role
Out[2]: <enum ATSPI_ROLE_APPLICATION of type Atspi.Role>
In [3]: acc.get_child_at_index(1)
Out[3]: <Atspi.Accessible object at 0x7f80888bcac0 (AtspiAccessible at 0x3df3250)>
In [4]: acc.get_child_at_index(1).name
Out[4]: ''
In [5]: acc.get_child_at_index(1).role
Out[5]: <enum ATSPI_ROLE_TREE of type Atspi.Role>
In [6]: acc.get_child_at_index(1).parent
In [7]: acc.get_child_at_index(1).parent == None
Out[7]: True