During `layout`, we now pass a mutable reference to the element's
parent view. This is a recursive process that causes the view to
be removed from `AppContext` and then re-inserted back into it once
the layout is complete.
As such, querying parent views during `layout` does not work as such
views will have been removed from `AppContext` and not yet re-inserted
into it. This caused a bug in `KeystrokeLabel`, which used the `keystrokes_for_action`
method to query its ancestors to determine their type id and keymap context.
Now, any time a view notifies, we will cache its keymap context so that
we don't need to query the parent view during `layout`.
Antonio Scandurra
18e39ef2fa