Re[5.6->5.15] SystemTrayIcon::isSystemTrayAvailable() crashes in certain situation.

If QSystemTrayIcon::isSystemTrayAvailable() is checked before QApplication is instantiated , then program crashed. Same used to work with older version of Qt. 

This breaks following use case to use QCoreApplication or QAappliction. 

"There is a command-line argument that will specify whether to start a gui or non-gui instance of the program. The gui parts of the program are entirely associated with the SystemTray. If the user specifies a gui instance, we check if the SystemTray is available. If it is not, we still proceed with a non-gui QCoreApplication.":

Since isSystemTrayAvailable() is static function it should work.