show more scenarios in choose_level debug command

the old code didn't work well for mp campaigns that dynamicially set the
next scenario in endlevel. The new code just shows all scenarios that
belong to the same addon, since this is a debug command it better to sho
a little too much than not to show all scenarios of the campaign.

src/menu_events.cpp

@@ -1668,14 +1668,17 @@ void console_handler::do_choose_level()
 	} else {
 		// find scenarios of multiplayer campaigns
 		// (assumes that scenarios are ordered properly in the game_config)
-		std::string scenario = menu_handler_.pc_.get_mp_settings().mp_scenario;
-		for(const config& mp : menu_handler_.game_config_.child_range("multiplayer")) {
-			if(mp["id"] == scenario) {
-				const std::string& id = mp["id"];
-				options.push_back(id);
-				if(id == menu_handler_.gamedata().next_scenario())
-					next = id;
-				scenario = mp["next_scenario"].str();
+		std::string scenario_id = menu_handler_.pc_.get_mp_settings().mp_scenario;
+		if(const config& this_scenario = menu_handler_.game_config_.find_child(tag, "id", scenario_id)) {
+			std::string addon_id = this_scenario["addon_id"].str();
+			for(const config& sc : menu_handler_.game_config_.child_range(tag)) {
+				if(sc["addon_id"] == addon_id) {
+					std::string id = sc["id"];
+					options.push_back(id);
+					if(id == menu_handler_.gamedata().next_scenario()) {
+						next = id;
+					}
+				}
 			}
 		}
 	}