Don't hard code reference formatter in tabbed view custom sorter.

Don't hard code reference formatter in tabbed view custom sorter. Look it up
from `IReferenceNumberSettings` instead.

This fixes the problem that caused the following traceback:
```
2014-03-07 10:39:29 ERROR Zope.SiteErrorLog 1394185169.020.129264338005 http://localhost:8080/mandant1/ordnungssystem/fuehrung/strategie-und-planung/@@tabbed_view/listing
Traceback (innermost last):
  Module ZPublisher.Publish, line 126, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 46, in call_object
  Module ftw.tabbedview.browser.tabbed, line 152, in listing
  Module ftw.tabbedview.browser.listing, line 109, in __call__
  Module ftw.tabbedview.browser.listing, line 198, in update
  Module opengever.tabbedview.browser.tabs, line 85, in custom_sort
  Module opengever.base.reference_formatter, line 128, in sorter
ValueError: need more than 1 value to unpack
```