Always buy the cheapest options in the cities view

When buying production with multiple cities selected in the cities view,
always buy from the cheapest cities. This ensures that we buy from as
many cities as possible.

Closes longturn#2311.

client/views/view_cities.cpp

@@ -441,10 +441,16 @@ void city_widget::clear_worlist()
  */
 void city_widget::buy()
 {
-  const auto saved_selection =
-      selected_cities; // Copy to avoid invalidations
+  auto saved_selection = selected_cities; // Copy to avoid invalidations
+
+  // Sort cheapest first
+  std::sort(saved_selection.begin(), saved_selection.end(),
+            [](const city *lhs, const city *rhs) {
+              return lhs->client.buy_cost < rhs->client.buy_cost;
+            });
+
+  // Buy
   for (auto *pcity : saved_selection) {
-    Q_ASSERT(pcity != nullptr);
     cityrep_buy(pcity);
   }
 }