squeezer: Add the xalign and yalign properties

This also centers the children by default instead of aligning them to
the start and the top when they are larger than the squeezer, as the
default xalign and yalign values are 0.5.

Fixes GNOME/libhandy#226
11 jobs for squeezer-align in 36 minutes and 54 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed api-visibility #831616

00:00:55

passed build-debian-clang #831618

00:04:17

passed build-debian-gcc #831617

00:09:12

passed build-fedora-gcc #831619

00:19:08

passed build-flatpak #831620
flatpak

00:03:18

passed build-flatpak-glade #831621
flatpak

00:05:00

 
  Test+Docs
passed build-gtkdoc #831622

00:04:40

passed unit-test #831623

00:03:06

44.8%
 
  Package
passed package-debian-buster #831624

00:10:06

 
  Test Package
passed autopkgtest-debian-buster-package #831625

00:02:57

passed lintian-debian-buster-package #831626

00:01:09