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 phosh#218 (closed)
Signed-off-by: Alexander Mikhaylenko email@example.com