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)
Status Name Job ID Coverage
  Build
passed build-debian-clang #235223
librem5

00:04:10

passed build-debian-gcc #235221
librem5

00:03:20

passed build-debian-gcc-static #235222
librem5

00:02:48

passed build-fedora-gcc #235224
librem5

00:03:13

 
  Docs
passed build-gtkdoc #235225
librem5

00:02:40

 
  Test
passed unit-test #235226
librem5

00:02:28

44.2%
 
  Package
passed package-debian #235227
librem5

00:02:26