preferences-window: Use HdyStatusPage for the empty search

This makes the code simpler and UIs look more cohesive.
11 jobs for preference-search-status-page in 15 minutes and 35 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed api-visibility #1047565

00:00:57

passed build-debian-clang #1047567

00:06:42

passed build-debian-gcc #1047566

00:02:17

passed build-fedora-gcc #1047568

00:04:06

passed build-flatpak #1047569
flatpak

00:01:24

passed build-flatpak-glade #1047570
flatpak

00:03:21

 
  Test+Docs
passed build-gtkdoc #1047571

00:02:22

passed unit-test #1047572

00:02:21

46.1%
 
  Package
passed package-debian-buster #1047573

00:03:34

 
  Test Package
passed autopkgtest-debian-buster-package #1047574

00:02:56

passed lintian-debian-buster-package #1047575

00:00:54