fullscreen-box: Check for autohide on is_fullscreen changes

Also use a setter instead of a signal handler and skip the setter if the
value didn't change.
2 jobs for master in 39 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #387464
flatpak:app

00:11:06

 
  Test
passed #387465
flatpak:tests

00:28:49