leaflet: Add the .folded and .unfolded style classes

This will help styling the children of the leaflet depending on whether
it is folded or not.

Fixes Librem5/libhandy#182.
12 jobs for folded-style in 31 minutes and 27 seconds (queued for 16 minutes and 36 seconds)
Status Name Job ID Coverage
  Build
passed api-visibility #239802
librem5

00:03:06

passed build-debian-clang #239805
librem5

00:04:35

passed build-debian-gcc #239803
librem5

00:03:22

passed build-debian-gcc-static #239804
librem5

00:03:14

passed build-fedora-gcc #239806
librem5

00:03:55

 
  Test+Docs
passed build-gtkdoc #239807
librem5

00:03:07

passed unit-test #239808
librem5

00:03:45

44.5%
 
  Package
passed package-debian-buster #239809
librem5

00:03:37

passed package-debian-buster:arm64 #239810
librem5:arm64

00:06:29

passed package-flatpak #239811
flatpak

00:01:52

 
  Test Package
passed autopkgtest-debian-buster-package #239812
librem5

00:03:15

passed lintian-debian-buster-package #239813
librem5

00:01:11