This is necessary for GtkNotebook so that the stack is always drawn
before the header.
And that is necessary so that the active tab can use negative
margins to overdraw the border of the stack to create a gap and
simulate old GTK2-style visuals for notebooks.
So far, the box gadget is always allocating all children the
full size in the cross axis. This behavior corresponds to the
align-items: stretch behavior in
https://www.w3.org/TR/css-flexbox-1/#align-items-property
This commit implements the other modes described there.
While widgets have halign/valign properties that we can use for
this, the API for inserting gadgets has to change to take an
extra align parameter. All callers have been updated to pass
GTK_ALIGN_FILL, since that corresponds to the previous behavior.
https://bugzilla.gnome.org/show_bug.cgi?id=760668