ToolItems are widgets that can appear on a toolbar.
To create a toolbar item that contain something else than a button, use ToolItem. Use add to add a child widget to the tool item.
For toolbar items that contain buttons, see the ToolButton, ToggleToolButton and RadioToolButton classes.
See the Toolbar class for a description of the toolbar widget, and ToolShell for a description of the tool shell interface.
menu_item_id
matches the string passed to
set_proxy_menu_item return the corresponding
MenuItem.
tool_item
about whether the item should appear in the toolbar overflow menu.