Fix dragging and dropping widgets internally.

Veusz version: 1.19
Python version: 2.7.1 (r271:86832, Mar 18 2011, 18:43:59)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-50)]
Python platform: linux2
Numpy version: 1.5.1
Qt version: 4.8.4
PyQt version: 4.10
SIP version: 4.14.4
Date: Wed, 20 Nov 2013 17:21:01 +0000

Traceback (most recent call last):
  File "/home/jss/veusz/build/veusz_pyinst/out00-PYZ.pyz/veusz.windows.widgettree", line 349, in dropEvent
  File "/home/jss/veusz/build/veusz_pyinst/out00-PYZ.pyz/veusz.windows.widgettree", line 327, in handleInternalMove
  File "/home/jss/veusz/build/veusz_pyinst/out00-PYZ.pyz/veusz.document.mime", line 103, in getWidgetMime
AttributeError: 'builtin_function_or_method' object has no attribute 'formats'