Option to use KNotifications
By setting the OPENTODOLIST_WITH_KNOTIFICATIONS to ON when calling cmake, the app can be build against KNotifications. This will cause it to use KStatusNotifierItem instead of QSystemTrayIcon, which is way more powerful.
For now, we enable this only for the Flatpak build, as there it allows us to revert back to using "KDE" features (before, we force the app to think it is running in GNOME to prevent issues with the tray icon).
Closes #370 (closed)