Bug#867794: python3-pyqt5: App tray icon won't show if show() before systray is available

[Jérôme <jerome@jolimont.fr>]

Thank you Dmitry for confirming and opening upstream bug in the right 
place.

Indeed, I intend to loop using a QTimer until 
QSystemTrayIcon.isSystemTrayAvailable().

--
Jérôme