Fix promote_tab broken for import plugins.

Veusz version: 1.20.1
Python version: 2.7.6 (v2.7.6:3a1db0d2747e, Nov 10 2013, 00:42:54)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]
Python platform: darwin
Numpy version: 1.7.0
Qt version: 4.8.5
PyQt version: 4.10.3
SIP version: 4.15.3
Date: Wed, 02 Apr 2014 19:25:06 +0000

Traceback (most recent call last):
  File "veusz/windows/mainwindow.pyc", line 709, in slotDataImport
  File "veusz/dialogs/importdialog.pyc", line 112, in __init__
NameError: global name 'ImportTabPlugins' is not defined