Fix #14994; that bug was introduced in #14228 (that was my fault).
~Also this commit explicit separates~ There are two functions of the tray icon:
- a system tray widget (
QSystemTrayIcon::isSystemTrayAvailable() == true) - a high-level notificator via balloon messages (
QSystemTrayIcon::supportsMessages() == true)
~These properties are mutually independent,~ e.g., on Fedora 29 + GNOME:
QSystemTrayIcon::isSystemTrayAvailable() == false;
QSystemTrayIcon::supportsMessages() == true;
UPDATE:
supportsMessages() makes no sense without isSystemTrayAvailable(): QSystemTrayIcon::showMessage() just not working on Fedora 29 + GNOME.
