Previous attempts to use auto exclusive zone from gtk-layer-shell failed
because gls was expecting real booleans (`TRUE`/`FALSE`) as set_anchor
arguments. With that being fixed, gtk_layer_auto_exclusive_zone_enable
makes gls handle everything related to the bar resizing.
The only remaining purpose of onConfigureGLS is to log warnings and bar
size changes; gtk-layer-shell code path no longer needs saved width_ or
height_ values.