elementary: destroy fileselector children when they are not itemized yet.

elementary: destroy fileselector children when they are not itemized yet.

As we now do everything asynchronously, we do have model representing child
of the main model that don't provide enough information to be displayed yet.
This are not tracked by a genlist item, nor are they a child of the
fileselector. To properly handle their lifecycle, it is necessary to unref
them manually explicitely.

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D8375