Casts the function pointer to an Etk_Signal_Callback_Function
Referenced by etk_combobox_build(), etk_combobox_entry_item_insert_empty(), etk_combobox_item_insert_empty(), etk_dialog_button_response_id_set(), etk_entry_clear_button_add(), etk_entry_image_highlight_set(), etk_mdi_area_put(), etk_toolbar_orientation_set(), etk_tooltips_tip_set(), and etk_window_modal_for_window().