paginator-box: Register window before setting parent

Prevents newly created widgets from reusing parent's window.

Fixes a regression from e6a47749

Fixes Librem5/libhandy#165

Signed-off-by: Alexander Mikhaylenko <alexm@gnome.org>
7 jobs for wip/exalm/paginator-parent in 10 minutes and 59 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-debian-clang #232210
librem5

00:03:23

passed build-debian-gcc #232208
librem5

00:02:33

passed build-debian-gcc-static #232209
librem5

00:02:29

passed build-fedora-gcc #232211
librem5

00:02:52

 
  Docs
passed build-gtkdoc #232212
librem5

00:01:45

 
  Test
passed unit-test #232213
librem5

00:01:32

44.0%
 
  Package
passed package-debian #232214
librem5

00:02:17