public static Actions::preRenderActionsDropbutton(&$element, FormStateInterface $form_state, &$complete_form)
#pre_render callback for #type 'actions'.
This callback iterates over all child elements of the #type 'actions' container to look for elements with a #dropbutton property, so as to group those elements into dropbuttons. As such, it works similar to #group, but is specialized for dropbuttons.
The value of #dropbutton denotes the dropbutton to group the child element into. For exa