Work around some mro issues with Pyside and QDialog

This "fixes" nucleic/enaml#108.  It seems a hack to me but the mro issues are beyond my abilities.  My understanding of the issue is

* QWindowDialog inherits from both QDialog and QWindowBase.  This gives a diamond inheritance pattern at QWidget but I'm not sure if that's the problem. 
* The layout setup happens in ```QWindowBase.__init__```
* In ```PyQt: QDialog.__init__``` is a ```<slot wrapper '__init__' of 'sip.simplewrapper' objects>``` but ```somehow the QWindowBase __init__``` is called. 
* In ```PySide: QDialog.__init__``` is a ```<slot wrapper '__init__' of 'PySide.QtGui.QDialog' objects>``` and ```QWindowBase.__init__``` is not called. 

Trying in PySide to only call QWindowBase.__init__ led to seg. faults.