box gadget: Implement cross-axis alignment

box gadget: Implement cross-axis alignment

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