boxlayout: Compute opposite size properly

boxlayout: Compute opposite size properly

For size -1 in the opposite orientation, GtkBoxLayout used to measure
the children based on their min size in the box's orientation instead of
-1. That wasn't really intended, but was a side effect of how the sizing
code did (not) distribute extra size above the minimum size.

This is clearly not what we want.
What we want is measuring the orientation as is for size -1. Then we
want to just take the maximum of all children and use that.

A reftest is incldued that ensures a vbox wraps a label just like an
hbox does.