box gadget: Fix allocation without baseline

box gadget: Fix allocation without baseline

When we don't get a baseline passed in, we want to basically
center the children inside the allocation. There was an attempt
in the code to do 'internal baseline alignment', but it had the
side effect of moving the contents to the top when we don't get
a baseline passed in. Remove it for now, this needs some more
infrastructure to do properly.

https://bugzilla.gnome.org/show_bug.cgi?id=761363