Drop compatibility code for ancient GTK versions
GTK 3.18 was released in September 2015 and is the first version to support the Keyboard Shortcuts overlay.
Since that's our minimum version, we can drop code for older GTK releases.
GTK 3.18 was released in September 2015 and is the first version to support the Keyboard Shortcuts overlay.
Since that's our minimum version, we can drop code for older GTK releases.