Check for system tray icon

This isn't really needed because we always have a system tray icon, but
when investigating why Kleopatra doesn't quit when the main window is
closed even though quitOnLastWindowClosed was true I temporarily disabled
the creation of the system tray icon. And it doesn't hurt to check.

(cherry picked from commit 7e4da3c73fd523751af0e2434872564a42550a4c)