mouse: Don't use gtk_widget_show_all

It is removed in GTK+ 4
11 jobs for <span class="ref-name">mouse-no-show-all</span> in 3 minutes and 32 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #114412
build

00:01:53

 
  Test
passed #114414
asan

00:01:36

passed #114413
test

00:00:24

 
  Manual
manual #114419
allowed to fail manual
armv8
manual #114415
allowed to fail manual
flatpak
manual #114418
allowed to fail manual
msan
manual #114420
allowed to fail manual
ppc64le
manual #114416
allowed to fail manual
tsan
manual #114417
allowed to fail manual
ubsan
 
  Review
manual #114421
allowed to fail manual
review
manual #114422
allowed to fail manual
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