GTK: split out the computation part from set_geom_hints.

GTK: split out the computation part from set_geom_hints.

This is preparation for dealing with the fact that GTK's geometry-
based API routines for setting the window size are being deprecated:
we'll no longer be able to specify a width/height in characters and
have GTK convert that into a pixel size based on the geometry hints
we'd already fed it. So we'll need to do that conversion ourselves,
and the easiest approach is to make it easy to recompute the geometry
hints on our side whenever we need them.