paginator-box: Stop using gtk_widget_set_child_visible()

This function is meant for widgets that don't need to be mapped along
with parent widget, not for scrolled out widgets. Additionally, using it
causes strange side effects with GtkOverlay window z-ordering. Stop using
it and instead track visiblity manually. Also, clarify the code a bit.

Fixes Librem5/phosh#218
7 jobs for wip/exalm/visible in 17 minutes (queued for 3 minutes and 44 seconds)