st/bin: Delegate child allocation to layout manager

st/bin: Delegate child allocation to layout manager

Since commit ed3b8d093e16, the actor's single-child policy is
enforced even when using the generic add/remove_child() methods.

This makes it possible to follow GTK/AdwBin's lead and replace
out custom child allocation with a BinLayout, without changing
the actor's semantics in a fundamental way.

However this does change slightly how the child is allocated, as
it now only expands according to its expand properties, not when
the alignment is set to FILL.

Even after the preparations in the previous commit, some fallout
is still likely, but it will be easy to address, and the cleanup
is significant enough to justify it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3046>