preferences-page: Implement the margins via CSS

This makes the widget more flexible.
11 jobs for preference-group in 15 minutes and 57 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed api-visibility #1051400

00:00:14

passed build-debian-clang #1051402

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passed build-debian-gcc #1051401

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passed build-fedora-gcc #1051403

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passed build-flatpak #1051404
flatpak

00:07:12

passed build-flatpak-glade #1051405
flatpak

00:03:08

 
  Test+Docs
passed build-gtkdoc #1051406

00:02:22

passed unit-test #1051407

00:02:24

46.0%
 
  Package
passed package-debian-buster #1051408

00:03:34

 
  Test Package
passed autopkgtest-debian-buster-package #1051409

00:02:45

passed lintian-debian-buster-package #1051410

00:00:59