preferences-window: Strictly center the header bar

This makes the header bar's widgets look better by ensuring they are
always centered, even if it means they will be narrower.
7 jobs for preferences-strictly-center in 14 minutes and 54 seconds (queued for 2 minutes and 21 seconds)
Status Job ID Name Coverage
  Build
passed #48442
librem5
build-debian-clang

00:02:37

passed #48440
librem5
build-debian-gcc

00:02:44

passed #48441
librem5
build-debian-gcc-static

00:01:45

passed #48443
librem5
build-fedora-gcc

00:03:59

 
  Docs
passed #48444
librem5
build-gtkdoc

00:01:39

 
  Test
passed #48445
librem5
unit-test

00:02:59

45.8%
 
  Package
passed #48446
librem5
package-debian

00:02:03