GtkAccelLabel: adjust recent accel changes
In the event that a GtkAccelKey was present for the closure but it contained a keyval of 0 the previous code would show "". After the recent adjustments, "-/-" would be shown in this case. It turns out to be a pretty common case, so fix the logic to stop using '0' as a magic value to mean "don't have an accel" and add a separate boolean for that purpose.
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