Cache the computed border size so we can fetch the border size at
any time without worrying that we'll be spending too much time in
the theme code (in some cases we might allocate a PangoFontDescription
or do other significant work.)

The main effort here is clearing the cache when various bits of window
state change that could potentially affect the computed borders.

https://bugzilla.gnome.org/show_bug.cgi?id=707194