BUG : modified logic on starting qApp

when working in ipython, it seems that _create_qApp is not called
until the first time that a figure is made, but ipython has already
started up the QApplication for us (I think) so that we never set
backend_qt5 to be non-None.  This seems to mostly be ok, but breaks
`plt.pause`.
Merge pull request #3360 from tacaswell/qApp_fix

BUG : modified logic on starting qApp