gtk: Have GtkCenterLayout hold a reference to its widgets
These setters don't ref the passed widget. This was found by the gtk-rs-fuzzer: https://github.com/gtk-rs/gtk4-rs/issues/926
Edited by Jason Francis
These setters don't ref the passed widget. This was found by the gtk-rs-fuzzer: https://github.com/gtk-rs/gtk4-rs/issues/926