DEV: allows categories-admin-dropdown to have custom items (#9931)

Usage:

```
modifySelectKit("categories-admin-dropdown").appendContent(function() {
  return {
    id: "foo",
    name: I18n.t("foo"),
    description: I18n.t("foo_description"),
    icon: "gear",
    onChange: () => alert("Foo.")
  };
});
```

app/assets/javascripts/select-kit/addon/components/categories-admin-dropdown.js

@@ -36,5 +36,13 @@ export default DropdownSelectBoxComponent.extend({
     }
 
     return items;
-  })
+  }),
+
+  _onChange(value, item) {
+    if (item.onChange) {
+      item.onChange(value, item);
+    } else if (this.attrs.onChange) {
+      this.attrs.onChange(value, item);
+    }
+  }
 });