[PyQt] Keep references to window modal, native dialogs on OSX, Qt5.3

[lloyd konneker]

I had symptoms on OSX10.9, Qt5.3 where dialogs closed themselves, and 
print dialog segfaulted.  I found that I was not keeping a reference to 
the dialog, beyond the call to dialog.open().  I suppose this is 
especially important where Qt enlists native dialogs, such as 
QMessageBox and QPrintDialog, and when you open the dialogs as sheets 
(window modal.) I can't remember this specific problem on earlier 
versions of OSX and Qt, so I imagine its a subtle change in the sequence 
of events.  Anyway, keep a reference, and port early and often.