app, themes: fix display-shell statusbar height

In gimp.css, don't set a minimum height for GimpDisplayShell
statusbars.  Instead, in GimpStatusbar, set the widget's minimum
height to the maximum of its children's natural heights.  Note that
we have to do this manually, instead of using a size group, since
GtkSizeGroup::ignore-hidden is deprecated (and nonfunctional) in
GTK3.
12 jobs for master in 24 minutes and 46 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #772885
build-image

00:02:24

passed #772887
build-image-win32

00:01:14

passed #772886
build-image-win64

00:02:04

 
  Dependencies
passed #772888
deps-debian/testing

00:02:33

passed #772890
deps-win32

00:07:01

passed #772889
deps-win64

00:12:28

 
  Gimp
passed #772891
gimp-debian/testing-autotools

00:10:39

passed #772893
gimp-debian/testing-clang

00:13:08

passed #772892
gimp-debian/testing-meson

00:03:24

passed #772895
gimp-win32

00:13:47

passed #772894
gimp-win64

00:10:10

 
  Analysis
passed #772896
cppcheck

00:02:28