Forbid toggling the minimap from the city view

It's never visible, so the menu would do nothing. The implementation is also
buggy when triggered while the widget is hidden.

Closes longturn#1126.

client/citydlg.cpp

@@ -1475,6 +1475,7 @@ void city_dialog::hideEvent(QHideEvent *event)
     pcity = nullptr;
   }
   queen()->mapview_wdg->show_all_fcwidgets();
+  king()->menu_bar->minimap_status->setEnabled(true);
 }
 
 /**
@@ -1487,6 +1488,7 @@ void city_dialog::showEvent(QShowEvent *event)
   if (pcity) {
     unit_focus_set(nullptr);
     update_map_canvas_visible();
+    king()->menu_bar->minimap_status->setEnabled(false);
   }
 }