st/box-layout: Fix forwarding `vertical` changes
Quoting the main commit:
st/box-layout: Fix forwarding `vertical` changes
Our code to "forward" `notify` signals from the underlying layout
manager for the `vertical` property broke when ClutterLayoutManager
removed its own `vertical` property.
Property changes via the underlying layout manager should be rare,
but still worth fixing, so explicitly connect to `notify::orientation`
now.