gtk-clients' behavior is really bad when there's no action enablers for unit upgrade in the ruleset. The upgrade buttons are still sensitive, and clicking one even brings up the confirmation dialog with the assumed price of the upgrade. Make those upgrade buttons insensitive with such rulesets.
Split from #42574 reported by ihnatus.
gtk-clients' behavior is really bad when there's no action enablers for unit upgrade in the ruleset. The upgrade buttons are still sensitive, and clicking one even brings up the confirmation dialog with the assumed price of the upgrade. Make those upgrade buttons insensitive with such rulesets.