add 'onselect' callback & remove 'default' tag

src/graphics/ui/elements/SelectBox.hpp

@@ -14,6 +14,7 @@ namespace gui {
     private:
         std::vector<Option> options;
         Option selected {};
+        StringCallbacksSet changeCallbacks;
     public:
         SelectBox(
             GUI& gui,
@@ -23,6 +24,8 @@ namespace gui {
             const glm::vec4& padding
         );
 
+        void listenChange(onstringchange&& callback);
+
         void setSelected(const Option& selected);
 
         const Option& getSelected() const;