Function traversal 

pub fn traversal(el: Div) -> Div

Attach the traversal handlers, normally to the app’s root element.

It has to live on an element rather than on the app because moving focus needs a Window, and an app-level action handler only gets an App.

Where the focused surface claims the key, both handlers propagate instead: an action handler stops propagation by default, and continuing it is what sends gpui on to the next binding the chord matched.