Creates a new context menu item. If an error occurs during creation, it
may not be detected until the creation callback fires; details will be in
runtime.lastError.
callback Called when the item has been created in the browser. If an
error occurs during creation, details will be available in
runtime.lastError.
returns The ID of the newly created item.
Updates a previously created context menu item.
id The ID of the item to update.
updateProperties The properties to update. Accepts the same values as
the contextMenus.create function.
returns Called when the context menu has been updated.