view-switcher: Fix stack children callbacks

This fixes the callbacks when a child is added or removed from the view
switcher's stack.
7 jobs for view-switcher in 8 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #23682
librem5
build-debian-clang

00:02:20

passed #23680
librem5
build-debian-gcc

00:02:38

passed #23681
librem5
build-debian-gcc-static

00:01:54

passed #23684
librem5
build-debian-package

00:01:43

passed #23683
librem5
build-fedora-gcc

00:04:05

 
  Docs
passed #23685
librem5
build-gtkdoc

00:01:28

 
  Test
passed #23686
librem5
unit-test

00:02:28

38.9%