This PR fixes #178 by decoupling the system tray menu from the system tray icon itself: in the event that the application is being run under a desktop environment that does not support a system tray (such as the latest vanilla version of GNOME), a gear/cog button will appear in the bottom-right corner of the status panel which, when clicked, will display the menu that was previously inaccessible.

For users of desktop environments that do support a system tray (including macOS, Windows, Ubuntu and, seemingly, every other commonly-used Linux distro/environment), this changes nothing: the menu will continue to be accessible from the system tray, as expected.