window: Hide headerbar buttons that can't be used

Instead of changing their sensitivity

Fixes #626
8 jobs for wip/sadiq/empty-states in 9 minutes and 58 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-pureos-gcc-byzantium #377159
librem5

00:03:14

passed build-pureos-gcc-byzantium-no-purple #377158
librem5

00:01:44

 
  Test
passed check-po #377161
librem5

00:00:20

passed test:debian-gcc #377160
librem5

00:01:39

18.9%
 
  Package
passed package-pureos-byzantium #377162
librem5

00:01:40

passed package-pureos-byzantium:arm64 #377163
librem5:arm64

00:05:03

 
  Test Package
passed autopkgtest-pureos-byzantium-package #377164
librem5

00:01:17

passed lintian-pureos-byzantium-package #377165
librem5

00:00:44