ClutterBoxLayout's layout policy of using the generic ClutterActor
align/expand properties for children that are expanded and a custom
meta otherwise is confusing, in particular as the x-fill/y-fill
defaults don't match the default CLUTTER_ACTOR_ALIGN_FILL align.
StBoxLayout's own custom child meta (which was deprecated last
cycle) is probably the only consumer. And luckily, the St meta
uses different x-fill/y-fill default that match the ClutterActor
defaults, so removing it will not affect code that doesn't use
the deprecated properties themselves.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1265
11f224f4b0