feat(dropdown): keep the list on open/close for simpler styling

So far it was deleted on close, now it's just hidden/unhidden

src/widgets/lv_dropdown.h

@@ -226,6 +226,12 @@ void lv_dropdown_open(lv_obj_t * dropdown_obj);
  */
 void lv_dropdown_close(lv_obj_t * obj);
 
+/**
+ * Tells whether the list is opened or not
+ * @param obj       pointer to a drop-down list object
+ * @return          true if the list os opened
+ */
+bool lv_dropdown_is_open(lv_obj_t * obj);
 
 /**********************
  *      MACROS