Rework window geometry to avoid using geometry_widget.

Rework window geometry to avoid using geometry_widget.

Partly this is because the geometry_widget functionality is going away
in a later version of GTK3, so sooner or later we'll need not to be
using it anyway. But also, it turns out that GTK 3's geometry
calculations have the unfortunate effect of setting the window's base
and min heights to values that are not congruent mod height_increment
(because the former is the value we gave, but the latter is based on
the minimum height of the scrollbar), which confuses at least one
window manager (xfwm4) and causes the window to be created one row too
small.

So I've redone all the geometry computations my own way, based on the
knowledge that the only widgets visible in the top-level window are
the drawing area and the scrollbar and I know how both of those
behave, and taking care to keep base_height and min_height congruent
to avoid that xfwm4 bug.