The code used static_cast blindly even if the object not necessarily
was of the assumed subclass. Crash followed. Use dynamic_cast instead
to be able to check, like the corresponding code for GTK a11y. After
that fix we then got a nice infinite loop whn an object apparently is
its own parent. Detect that and break out.

Whether accessibility for the affected parts of the GUI actually works
now for its real use cases, no idea. But anything is better than a
crash...

Change-Id: I862234e168db0bd19d185b0f1f73b733b570cac4