restrict slider ui to slider fields

src/data/field/choice.hpp

@@ -40,6 +40,7 @@ public:
   String default_name;                      ///< Name of "default" value
   map<String,Color> choice_colors;          ///< Colors for the various choices (when color_cardlist)
   map<String,Color> choice_colors_cardlist; ///< Colors for the various choices, for in the card list
+  bool is_slider = false;                   ///< Should the UI be displayed as a slider?
 
   void initDependencies(Context&, const Dependency&) const override;
   void after_reading(Version ver) override;

src/data/field/slider.cpp

@@ -35,6 +35,8 @@ IMPLEMENT_REFLECTION(SliderField) {
 void SliderField::after_reading(Version ver) {
   Field::after_reading(ver);
 
+  is_slider = true;
+
   int choice_count = choices->choices.size();
 
   if (maximum < minimum) swap(minimum, maximum);

src/gui/value/choice.cpp

@@ -202,27 +202,21 @@ DropDownChoiceList::DropDownChoiceList(Window* parent, bool is_submenu, ValueVie
   : DropDownChoiceListBase(parent, is_submenu, cve, group)
 {
   // determine if slider
-  size_t count = itemCount();
-  if (count > 3) {
-    int value;
-    try {
-      String first_item = capitalize(itemText(0));
-      if (first_item != _("Default")) value = std::stoi(first_item.ToStdString());
-      value = std::stoi(itemText(1).ToStdString());
-      value = std::stoi(itemText(count - 2).ToStdString());
-      value = std::stoi(itemText(count - 1).ToStdString());
-      // the choices are numbers, use a slider
+  if (itemCount() > 1 && field().is_slider) {
     is_slider = true;
     // load slider images if needed
     if (!slider_loaded) {
       slider_loaded = true;
+      try {
         slider_left = load_resource_image(_("slider_left"));
         slider_right = load_resource_image(_("slider_right"));
         slider_center = load_resource_image(_("slider_center"));
         slider_tick = load_resource_image(_("slider_tick"));
       }
+      catch (...) {
+        throw InternalError(_("Can't load slider resources"));
+      }
     }
-    catch (...) {}
   }
 }