GtkQTreeModel: insert all children on rowsInserted

In the case of a QSortFilterProxy model, when rows which were
previously filtered are re-inserted, no rowsInserted signal is
emitted on their children. To handle this case, we need to
explicitly re-insert any children.

Change-Id: If46288356a8697477643671d142fe846aa656ae1
Tuleap: #651