window: Don't use set_allocation() to store cached values

window: Don't use set_allocation() to store cached values

The window size can be queried on widget->window directly, no need to
store it in widget->allocation.
This change is necessary because gtk_widget_set_allcation() is now
checking invariants that assume it's called from insize
gtk_widget_size_allocate() and that wasn;t the case here.