GTK3 port: replace size_request in the Columns layout class.

GTK3 port: replace size_request in the Columns layout class.

It's been replaced by a new pair of methods get_preferred_width and
get_preferred_height. For the moment, I've followed the porting
guide's suggestion of keeping the old size_request function as an
underlying implementation and having each of those methods just return
one of its outputs. The results are ugly, but it'll compile and run,
which is a start.