Do not always FILL to center
#8799 (closed) already fixed some alignment behavior but it misses one.
The align 0.5 is converted always in Gtk.Align.FILL but in some case we just want it to be Gtk.Align.CENTER when the widget is not expanded.
E.g.: an Entry next to a MultiSelection (which takes few lines). It should be centered but not fill all the spaces vertically.