datetime: Don't use gtk_widget_show_all

It is removed in GTK+ 4
11 jobs for datetime-no-show-all in 3 minutes and 47 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #114456
build

00:01:45

 
  Test
passed #114458
asan

00:02:01

passed #114457
test

00:00:35

 
  Manual
manual #114463
allowed to fail manual
armv8
manual #114459
allowed to fail manual
flatpak
manual #114462
allowed to fail manual
msan
manual #114464
allowed to fail manual
ppc64le
manual #114460
allowed to fail manual
tsan
manual #114461
allowed to fail manual
ubsan
 
  Review
manual #114465
allowed to fail manual
review
manual #114466
allowed to fail manual
stop_review