Qt TableEditor: `reorderable` does not work as expected

In the Qt TableEditor when setting:
- `reorderable=False`, dragging a row will through a warning

``` python
...\traitsui\traitsui\qt4\view_application.py:136: RuntimeWarning: Invalid return value in function QAbstractTableModel.flags, expected ItemFlags, got NoneType.
start_event_loop_qt4()
```
- `reorderable=True`, dragging a row will result in removing the row from the view (even when we set `deletable=False`).

**Notes**:
_You can verify this strange behaviour playing around with https://github.com/enthought/traitsui/blob/master/examples/demo/Standard_Editors/TableEditor_demo.py_