Do not call setQuitOnLastWindowClosed() on a QCoreApplication

[stukowski]

Fixes the following error.

Notebook cell:

%gui qt
from PyQt5 import QtCore
app = QtCore.QCoreApplication([''])

Kernel error:

Traceback (most recent call last):
  File "c:\users\astuk\appdata\local\programs\python\python38\lib\site-packages\tornado\ioloop.py", line 741, in _run_callback
    ret = callback()
  File "c:\users\astuk\appdata\local\programs\python\python38\lib\site-packages\ipykernel\kernelbase.py", line 402, in advance_eventloop
    eventloop(self)
  File "c:\users\astuk\appdata\local\programs\python\python38\lib\site-packages\ipykernel\eventloops.py", line 130, in loop_qt5
    return loop_qt4(kernel)
  File "c:\users\astuk\appdata\local\programs\python\python38\lib\site-packages\ipykernel\eventloops.py", line 120, in loop_qt4
    kernel.app.setQuitOnLastWindowClosed(False)
AttributeError: 'QCoreApplication' object has no attribute 'setQuitOnLastWindowClosed'

loop_qt4() currently expects the get_app_qt4() function to always return a QApplication object (or a QGuiApplication in Qt5). This is not a valid assumption. The user may be creating just a QCoreApplication base class instance to run Qt event processing in the backend, never displaying any Qt windows/widgets. Non-UI Qt applications do not support the setQuitOnLastWindowClosed() method.

[blink1073]

Thank you!