Codechange: Avoid emplace_back(new()) into a unique_ptr. (#11384)

This could theoretically leave an unmanaged pointer in certain circumstances, and directly using
make_unique shows intent.

src/game/game_gui.cpp

@@ -311,7 +311,7 @@ struct GSConfigWindow : public Window {
 
 							DropDownList list;
 							for (int i = config_item.min_value; i <= config_item.max_value; i++) {
-								list.emplace_back(new DropDownListStringItem(config_item.labels.find(i)->second, i, false));
+								list.push_back(std::make_unique<DropDownListStringItem>(config_item.labels.find(i)->second, i, false));
 							}
 
 							ShowDropDownListAt(this, std::move(list), old_val, WID_GSC_SETTING_DROPDOWN, wi_rect, COLOUR_ORANGE);