background: Don't use gtk_widget_show_all

It is removed in GTK+ 4
11 jobs for background-no-show-all in 4 minutes and 17 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #114445
build

00:02:04

 
  Test
passed #114447
asan

00:02:12

passed #114446
test

00:00:41

 
  Manual
manual #114452
allowed to fail manual
armv8
manual #114448
allowed to fail manual
flatpak
manual #114451
allowed to fail manual
msan
manual #114453
allowed to fail manual
ppc64le
manual #114449
allowed to fail manual
tsan
manual #114450
allowed to fail manual
ubsan
 
  Review
manual #114454
allowed to fail manual
review
manual #114455
allowed to fail manual
stop_review